Unfortunately theft is a growing problem in Asia, but by using a bit of common sense you'll most likely be able to avoid any problems.
Money belts are the most popular way to protect valuables like passports, travellers cheques and credit cards.
In looking for a money belt (they're mostly worn around the waist) look for one made of lightweight material (silk is good) that will not sweat up in the Asian heat and humidity. Also make sure the clasp is sturdy. Multiple zipper-sealed pockets are handy. Lastly, make sure that the money belt is reasonably easy for you to access -- there's nothing worse than a belt that needs to be taken off to be opened!
Money belts can also be purchased in a pouch style that is worn around the neck. While equally popular, we'd lean towards recommeneding a waist belt as they are more secure, if slightly less comfortable.
Good items to have are combination locks (no need to worry about lost keys -- just don't forget the combination) to lock as many of the backpack zippers as possible.
Pacsafe is an increasingly popular product made of a thin wire mesh webbing that totally covers your backpack and allows it to be padlocked to something secure. While it won't totally stop prying hands, it is a great way to secure your pack more than others. You can read more about Pacsafe here.
A general word on bag security. Don't ever, ever, ever pack items of value in your pack then store the bag out of sight (eg on the roof or in the cargo hold of a bus). Theft from stowed bags has become an endemic problem in Thailand -- particularly on the buses running from Khao San Road to Ko Samui, Ko Pha Ngan and Ko Tao. The easiest way to avoid this is to not take buses from Khao San Road and instead head to the government bus station where it is far less of a problem. But if you insist on using the Khao San buses, don't store anything of value in the luggage compartment.
Planning well is an integral part of getting the most out of your trip. Be it picking the right backpack, the right vaccinations or the right country, the simple decisions are often the most important.