A money belt is, in my honest opinion, a vital accessory for any traveler. Money belts are thin, zippered pouches on a small elastic belt that are designed to be worn underneath your pants/shorts/skirt, out of sight of potential thieves. The belts serve as storage for everything vital to your trip, including documents such as your passport and visas, any other forms of identification, and any pre-purchased airplane and train tickets. Your main sources of money should also be kept here, including any large amounts of cash, traveler's checks, and ATM/credit cards. The belt is much more secure than your luggage or backpack, which are easier to steal. Even if everything else is stolen or lost, your trip can still continue as long as you have your money belt.
Obviously, it is important to keep the money belt fully hidden from potential thieves while you are travelling. When wearing the money belt be sure to tuck in any straps that may hang out. It's also best to not fumble around in your money belt in a crowded area. I like to keep a day's supply of cash easily accessible in my front pocket or in a pocket that can latch shut so I can keep my belt safely tucked away. You can also leave your belt for the day in some hotel safes, but please use your common sense. I feel better erring on the side of caution and typically wear my belt at all times, only taking it off when I am in a secure, locked area like my hotel room. Losing your passport and return trip airplane tickets can make getting home a big headache.
A warning: any paper documents in the money belt can get damp from perspiration, especially for active travelers or those that are in hot areas. To avoid this, keep everything inside the belt wrapped in a plastic baggie.
Money belts tend to be one size fits all and relatively cheap, about $10 American dollars. Stores that sell travelling gear and luggage will probably be your best bet for finding money belts. When shopping around for one be sure to try it on and check that it is comfortable. Also take a good look at the clasp that holds the belt together; cheaper belts have flimsy fasteners that might break at inopportune times. Even though money belts are designed to be as thin and small as possible, many people still find them bulky and uncomfortable. For those that really dislike the idea of wearing one there are similar devices available that can be worn around the neck or strapped to the ankle.
Thank you, Rick Steves.