When travelling, you're probably naturally inclined to keep all of your stuff together, where you won't lose track of it. However, if something happens to that one bag, you'll have lost everything essential to your trip. Separate your valuable travel items and put them in several safe places -- passports, cash, traveler's checks, identification, plane tickets -- should all be in separate zipper pockets on your person or in separate pieces of luggage. If you get robbed or lose something, at least you won't have lost everything.
