The same way you keep your balance on the road. It's exactly the same physics, except the ground is moving instead of the bike moving. The tough part of learning to ride rollers is that you tend to drift from side to side and the rollers are only so wide. (2.5-3 feet maybe) The hardest part is starting the wheels moving, if you don't commit you're screwed. But, once you get the hang of it they're really the best way to ride indoors. I was able to ride no handed on the rollers after a while. But, they do take a little more concentration than a stationary trainer, so if you're easily distracted you might not want to risk your ass. (Yota)