What you bring depends on the length of the trip and how far off the beaten track you intend to go. It's also worth talking to people who have made similar journeys to get tips on indispensable items and things you can afford to leave behind.
The following list was compiled by www.usitnow.ie.
Pack: backpack; day pack; two small locks; bungee cord (for wrapping around door and rack on trains); sleeping sheet (thin and light); a large (lightweight) nylon stuff sack to put backpack in during flight - it protects the straps and cords from being damaged; water bottle; an extra buckle for straps on your backpack.
Documents: passport and visas; health card; hostel card; driver's licence (useful for security deposit on keys, etc.).
Health: first-aid kit with bandaids, tape and antibiotic ointment, antacid, cold medicine, vitamin C, anti-diarrhoea and anti-nausea tablets; extra pair of prescription glasses; toothpaste, brush, floss; nail clippers, tweezers; ear plugs, tissues, Q tips; suntan/sunburn lotion; razor; hand-wipes; shampoo/conditioner; hair ties; feminine hygiene products.
Equipment: compass; binoculars; small light; pocket knife; screwdriver, corkscrew and bottle opener; eating utensil, cup; small alarm clock; torch with spare batteries; camera and film; recorder and tapes (audio tapes are great to send home); batteries; small bag of detergent; small sewing kit; referee's whistle (a great item for a woman to have with her); pen, paper, journal; plastic bags (sandwich-size, ziplock) for separate currencies, leaking items, etc.; guidebook; books to read; language reference.
Clothing: windbreaker/ raincoat; two pairs of trousers (jeans or khakis); six pairs of socks; six pairs of underwear; two pairs of shoes (hiking boots/tennis shoes); two T-shirts (you will buy more as needed); one jumper; gloves; ear/headband or hat; two pairs of shorts; a bandana comes in handy lots of ways; plastic bags to keep wet or dirty clothes away from your other luggage.