If you rent a bicycle or motorcyle on your trip, avoid the temptation to put your day pack in the basket. You'll be much better off wearing the pack on your back instead.
The trick is that two guys will swing by on a motorcycle, with the passenger snatching your pack as they zip past and disappear.
Don't be dumb like me either, and tie the pack to the basket thinking that this will keep your stuff safe. I did that a few years ago with a rental bicycle in Vientiane, and while the guys weren't able to pull the pack loose from the bike basket, they did manage to drag my sorry butt for several meters along the asphalt street. Bummer.
Southeast Asia is still a relatively safe place, and these types of incidents are still fairly rare (it was the only time I ever had a problem in 6 years of travelling there), but if it happens to you, it could put a big hurt on your trip. Cheers.
I ws robbed in exactly that manner a couple of years ago in Krabi -- you don't expect to be robbed while riding along at 60km/h!