The most important thing about your bag is that you can carry it. A large backpack is best. Try some in the shop to see what feels comfortable – put some stuff in it, if you can, to see what it feels like full. One with several compartments is useful, especially if you can open them at the bottom. The ones that you can open like a suitcase are nice for finding your stuff, but not very comfortable to carry.
If you buy a smallish bag (40 litres or so), you won't be tempted to pack it full of stuff. A full 40-litre bag can fit about 12kg of stuff in it, which pretty much anyone can manage. Bigger and stronger people might want to carry up to 25kg, but then they might end up regretting it. Especially if it's over the airline's baggage allowance.
When considering your bag's size and weight, remember that you might end up carrying it quite a way, and you'll have to heave it into luggage racks, off baggage carousels etc. Don't fill it so full you can't lift it above your head.
A great piece of kit is a sports bag you can put your backpack into – and lock. These bags often double as a rain-cover too.
You will probably find you need a day sack as well, a small rucksack that serves as a carry-on bag, something to cart around a city etc. Again, one with lots of pockets and compartments is useful, especially if it has water bottle holders. The more space you leave in it, the easier it will be to find stuff when you're wedged into a tiny aeroplane seat. Be aware that in a city, your day sack is a bit vulnerable to thieves – try to get one with an inside pocket for your important things.
Roll your clothes, don't fold them. It means they don't get as crumpled. Heavy stuff is best on the bottom, and don't even think about taking or buying anything fragile! If you simply must, wrap it in your jumper or trousers, rather than wasting space with tissue paper.
Packing can be rather overwhelming, so why not start by deciding what not to take? Here's a list of things that could be restricted or illegal in your destination. Even if they're not, avoiding carrying such things will help your smooth transition through customs and immigration.
Passport Make sure there's at least a year left on it – some countries won't let you in otherwise.
Visas Obviously, these are stuck into your passport, but we've included them to help you remember to get them!
EHIC (if appropriate) You need this to get free healthcare in European countries.
Insurance documents Don't even think about travelling without it.
Driving licence Your British licence is usually OK for a foreign country, but double-check this if driving is going to be an essential part of your trip.
Tickets Hopefully obvious, but write down your ticket number and keep it separate, just in case.
Money A mixture of foreign currency, a debit or credit card, and travellers' cheques is usually advised. Avoid carrying around large amounts of cash – the commission or exchange rate you pay is usually worth the peace of mind.
Details of first night's accommodation Book at least one night before you leave, and keep the address and telephone number handy.
Photocopies of all documents plus contact details Just in case. Keep an electronic version too (scan them and email them to yourself) and write down important numbers (like your passport number or insurance claim line) to keep separately from the original.
Mobile phone and/or web-based email account so you can stay in touch with people. Check that your mobile will work in your destination, and buy an international sim card for cheaper rates. You might choose to buy an international calling card as well.
Underwear and socks Hopefully, the 'why' is fairly obvious! A week's worth is usually the right balance between not carrying loads, and not having to wash stuff all the time.
Swimwear Ladies might find a bikini more practical than a one-piece. Nothing too skimpy, mind.
Trousers/shorts with zippable pockets Lots of pockets are good, and zipped ones even better, as they keep your stuff nice and secure.
Long skirt and/or smart trousers If you're crossing a border, visiting an embassy, or get invited to dinner, you'll need something smart to wear. It doesn't have to be a suit, just something respectable. Don't even think about taking clothes that need ironing.
T-shirts Lots of them – they get dirty pretty quickly! Light colours keep you cool and deter mosquitoes, but dark ones hide dirt better. Vest tops are considered immodest (for both men and women) in some countries, particularly in places of worship.
Long-sleeved shirts (especially for women) Dressing modestly is a must in many places – and not just Muslim countries. Sometimes just covering shoulders and knees is enough, but a long-sleeved shirt is always handy. It helps keep the bugs and sun off your skin too.
Hat and/or scarf and/or gloves A hat is essential in both hot and cold weather. Ladies might choose to take a light scarf to cover their heads, especially in Muslim countries. It's not always necessary but it is an effective way of keeping the sun, bugs and dodgy men off you.
Jumpers and/or jacket Even if you're not planning on going anywhere cold, a light jumper is useful for over-airconditioned places, or as a cushion.
Waterproof These are useful for sitting on as well as wearing.
Shoes A pair of trek sandals and/or walking shoes are vital – even if you don't plan to do loads of walking, you'll probably end up tramping about more than you think.
Flip-flops, sandals or shower shoes In countries where they take their shoes off to go inside, anything that slips on and off easily is handy. Also, shower shoes are great on the beach as well as in skeevy hostels.
Something to sleep in Even if you don't normally bother, consider that you might be in a mixed dorm.
Moneybelt Waterproof ones are great, because you don't have to leave your stuff on the beach. You might want to take a cotton one too, as it lets your skin breathe.
Wedding ring Ladies may find a wedding ring helps deflect unwanted attention. Don't take a flashy or expensive one though.
Spongebag Waterproof, in case anything leaks.
Soap In a plastic box that doesn't leak.
Shampoo bar (and tin) Much lighter than normal shampoo because it doesn't contain water. It lasts longer too.
Flannel Useful as a compress as well as for washing.
Travel towel Smaller than a normal one, and they dry quicker too.
Toothbrush and toothpaste Don't rely on being able to buy decent ones where you're going.
Dental floss Dental floss makes an excellent thread for mending your clothes or backpack, because it's very strong. Also useful for when you get bits of pineapple stuck in your teeth.
Insect repellent Vital for anywhere hot, especially malaria areas. Get the strongest one you can – DEET is very effective, but some people don't like it because of safety concerns.
Sun screen (strongest you can find) Again, vital for hot places – for your health as much as anything else.
Moisturiser An all-purpose moisturiser is not just for the ladies – you can put it on after you shave, on your sunburn, on your feet when you've been walking all day.
Travel wash Multi-purpose travel wash is great. You can use it for your hair, clothes, body, and even food. It's usually concentrated so a little goes a long way. Most are also designed for camping so are biodegradable.
Universal plug Useful for doing your washing in the sink, or if you should be lucky enough to find a bath (even if a plug is provided, you won't always want to use it!). A universal plug can also be folded and wedged under your door in the absence of a deadbolt.
Condoms Hard to find in some places, and vital in areas where AIDS and hepatitis are endemic.
Tampons/sanitary towels (if appropriate) Again, hard to buy in some places, and can be expensive. The compact ones are best, as they take up less space and don't squash as easily.
Make-up (if appropriate) Even 'proper' travellers go to bars and clubs.
Hairbrush/comb/bands, hair product, nail clippers/file, razors and shaving cream For keeping yourself well-groomed.
Resealable plastic bags Useful for so many things – keeping your washing stuff in, keeping your snacks from the bugs, putting your wet swimming cossie in. Take loads.
Tissues and/or travel toilet paper 80% of the world's population don't use toilet paper. If you do, carry your own.
Baby wipes For when you're a long way from a sink.
Travel washing line Useful for hanging up your mosquito net, as well as your wet pants.
Travel alarm clock In case you have an early flight. Some have a light on which is handy.
Mosquito net Essential if you're going to a malaria area. Ones impregnated with insect repellent are best.
Sheet sleeping bag Hardly takes up any space, and is great if you've got doubts about the cleanliness of your hostel or guesthouse.
Earplugs To cut out the sound of the person in the next bed snoring, or the people behind you on the bus.
If you buy a travel first aid kit, lots of these things will be in it.
Plasters Take a variety of shapes and sizes, and make sure you get the kind that stick well.
Blister pads Wherever you are, you'll probably end up doing a lot of walking about. Blister pads are much better than plasters on your feet.
Water purification tablets You might not need them, but they're handy to have in emergencies. Get the all-in-one kind, rather than the ones you have to add neutraliser to.
Steroid cream and/or antiseptic cream or swabs For insect bites, scrapes and things. Mosquito and bedbug bites can be incredibly uncomfortable, and creams can ease your suffering.
Paracetamol/ibuprofen/aspirin Whatever you usually take when you feel pain.
Travel sickness pills and/or Sea-bands Even if you don't usually get travel sick, they're handy to have just in case – you might end up on a particularly rough boat trip, for example.
Malaria pills, if needed Your travel health advisor will talk over malaria medication with you – there are a number of different kinds of pills. Doxycycline are the most popular, and the cheapest.
Antihistamines If you are susceptible to allergies or hayfever – also useful for insect bites and skin rashes.
Sore throat sweets Air conditioning and planes can result in dry air which could give you a sore throat.
Rehydration powder and/or diarrhoea treatment Even if you're staying in the Western world, diarrhoea can happen, and if it does, you'll be thankful you brought something to treat it.
Prescription or over-the-counter medication, if needed You shouldn't need a doctor's note for most things, but you might want to double-check with your travel health advisor. If you have a specific condition (like diabetes), contact your support group for advice. If you're susceptible to certain things (eg mouth ulcers, stomach upsets) take your normal OTC treatment, just in case.
Dressings, tape and bandages Again, you might not need them, but they're very useful in an emergency.
Sterile needles Not everyone thinks these are necessary, but it's up to you. If you do decide to take them (eg in a sterile dental kit), don't mess about with them in any way – you don't want customs to get suspicious.
Guide book If you're going to more than one destination, only take your first guidebook. You should be able to buy the others there, or swap with travellers going the other way.
Notebook and pen/pencil For observations, writing down people's email addresses, etc.
Friendship cards Like business cards, but personal. Much easier than writing down the same information all the time.
Address book You can also print out your friends' names and addresses onto labels, to stick to postcards.
Book Just the one – you can swap it with another traveller or trade it in at a second-hand bookshop. Buy your magazines at the airport.
Postcards of your home town Useful as gifts for local kids, especially if they let you take their picture.
Playing cards For making friends and passing the time on boats, trains and places without telly.
Padlock and wire A wire like the kind you find on a bike lock is handy for chaining your bag to the luggage rack. Locking it not only stops people nicking stuff, but also stops them putting things into your bag.
Camera and batteries Digital ones are convenient for being able to email pictures to your friends right away. Think about what batteries or charger you'll need.
Sunglasses/glasses/contacts Take some spare specs if you're accident prone.
Torch The ones that fit on your head like a miners' lamp are the best, as they leave your hands free, and are good for reading and writing in bed.
Sewing kit For emergency repairs.
Penknife or multitool Fixing things, cutting up your mango, opening things.