The first thing to remember when packing for a long trip is to travel as light as possible. Remember that you may have to carry your backpack around for long periods; also, you may wish to buy things on your travels. Campsites have washing machines, so there is no need to carry a month's supply of clothes.
To make things easier on yourself when searching for things you've packed into your backpack, separate your luggage into see through bags. Zip lock plastic bags are especially good.
Also, bring plastic bags for dirty laundry, as well as for shower gels and shampoos, etc. (These may spill in your bag).
If you wear glasses, take a spare pair. Carry an adequate supply of any medicine you're on.
If travelling with a friend, buy one set of toiletries and split them. It sounds obvious but you could save an awful lot of space in you rucksack, and that space is precious.
Another way to save space is buy only one guide book even if you are visiting several countries on your trip. Buy a book for the first country you are visiting, when you are finished with that book; swap it with other backpackers, in a hostels library, or in a bookstore. Or simply buy them as you go.
Bring light-coloured, lightweight clothes, made from material that doesn't sweat or need ironing. Roll your luggage, it makes them easier to pack and helps prevent creasing. Bring one pair of good pants/jeans for going out as well as two tops. Don't bring anymore than this. Cargo pants that zip off at the knee are ideal for travelling, especially to hot countries. Bring a special microfiber travel towel as they are more practical than normal towels. Most specialist stores have them.
Bring Hiking boots, sandals, and flip flops (for showers in camp sites). Also, bring plenty of underwear and socks.
For more extensive lists visit our checklist page.