1. Set up next to a wall. Keep 1 hand on the handlebars and 1 on the wall. Ride and lift your wall hand off the wall...longer each time as you grow more comfortable. Eventually, you'll put that hand on the bars, too.
1a. Until you are comfortable, try it with regular shoes, and don't clip into your pedals.
2. Speed is your friend. It is actally easier to ride rollers fast, rather than slowly.
3. Look at a spot about 10' straight in front of you and imagine you are riding the bike towards it. That will help with holding your line.
4. As you get more comfortable. start to "explore" the edges of the rollers. I once rode right off my DH's rollers - okay, accidentally - and found myself.....upright and standing there by my bike. No big deal - no spectacular surge forward into the wall (or t.v.) or collapse to the ground. It made me realize that falling off rollers is not the end of the world.
5. If you absolutely cannot get the hang of rollers, but see and appreciate the value they offer in training, invest in a pair of parabolic rollers. I have a pair (not these) and love them. I call them my "cheater" rollers.