What you pack will depend on where you go and what time of year. The number one thing to keep in mind if you're backpacking is PACK LIGHT!! Bring one pair of VERY comfortable walking shoes (bad shoes can ruin a vacation). Sneakers are great, but they definitely signify "tourist." Don't bring shampoo for 3 months, it's easier and not a great expense to buy them as needed. Dress in layers to take more off your shoulders and to be prepared for a mix of weather. Bring shirts with sleeves and long pants (you'll look less like a tourist and many sacred places require shirtsleeves).

Travel Gear:
Backpack (how to choose a pack)
Day pack (for sightseeing during the day)
Hideaway pouch (belt or under shirt)
Cord & Small Locks (to tie luggage around doors and train racks)
Binoculars
First Aid Kit - Band-Aids, Antibiotic ointment, Antacid, Cold medicine

Papers:
Passport/Visas
Health Insurance Card
Youth Discount Card (ISIC - learn more)
Hostel Card
Address Book/Journal/Pens
Timetables/Itineraries/Maps
Tickets
Copies of documentation (store in a different bag from originals in case one gets lost)
Guidebook/Language Guide

Money:
ATM Card (check bank first for international access and to verify PINs)
Credit Cards (MasterCard and Visa are best bets)
Traveler's Cheques
Small calculator (to figure out exchanges and/or split costs with friends)

Basics:
Camera/Film/Batteries
Plastic bags (Ziploc - store leakable items or to save paperwork)
Sleep Sheets (how to make one)
Swiss Army knife/Can Opener
Matches

Clothes:
T-Shirts
Shorts/Pants/Jeans/Skirt
Socks (several pairs)
Belt
Shoes (make sure they are VERY comfortable)
FlipFlops (for showers & casually strolling)
Beachwear/Hat
Underwear
Semi-nice outfit
Sweater
Gloves/Hat/Thermal Underwear
Raincoat/Poncho/Umbrella
Sunglasses

Personal:
Toiletries: toothbrush, paste, floss, nail file/clippers, Q-tips, kleenex, razors, tweezers, shampoo/conditioner, soap, comb/brush, deodorant
Mirror
Sunscreen/moisturizers/cosmetics
Towel/washcloth
Condoms/Contraception
Medicine/Prescriptions

Extras:
CD Player & CDs
Collapsible Water Bottle
Alarm clock
Batteries
Sewing Kit (needle, thread, scissors, safety pins)
Small Bag of Detergent (to clean clothes/underwear)