For starters, all documents should be kept in a waterproof document holder (Heavy duty Ziploc bags are a cheap alternative).

It is best to keep all your original documents on you at all times, do not trust them to the hotel or hostel safe, especially in third-world countries.  Furthermore, never keep your documents in the same pocket as your money. In the unfortunate event of a mugging you can hand over your cash without endangering your far more precious documents. Keep photocopies of your documents in the hotel/hostel safe or in your main backpack, this is your backup in case you lose of all your originals. Do not keep your originals and photocopies in the same place!

Make sure you know the entry requirement for your destination in case they require special documents.

Original Documents:

• Airline Tickets
• Other Transportation Tickets
• Passport Photos
• Proof of Immunization
• Visas
• Passport
• International Student Card
• Proof of Medical Insurance
• Copy of Insurance Policy
• Travelers Cheques
• Checkbook
• Embassy Addresses
• Copy of Schedule
• Backup Photo ID (in case you lose your wallet)
• Credit Cards
• List of Contacts

Photocopies of the following:

• Airline Tickets
• Other Transportation Tickets
• Proof of Immunization
• Visas
• Passport
• International Student Card
• Proof of Medical Insurance
• Travelers Cheques
• Checkbook
• Credit Cards

A good tip is to print out this page and use it as a check-list while packing.