I want to train on rollers, and I figure once I get rolling it will be pretty easy, but how do I get my wheels turning fast enough to keep me upright before I fall over?
Get the floating rollers and you will be steady in about a minute.
Floating rollers look like this:
The rollers are attached to the inner frame which allows for rotation of the wheels and the inner frame is attached to an outer frame which allows forwards and backwards motion by the bicycle as a whole. This should help cyclists accelerate quicker than on traditional rollers, where there is a chance that you hop over the rollers and head forwards into the tv.
Started with mine in a door frame (since my house has no hallways) with shoulders aligned with the door jamb. Of course I didn't want it there or have to keep moving it. So not perfect.
The lasting solution... I have a spot in my basement where I attached a metal handle securely on the ceiling. Works much better to get started and just leave the roller there. I'm lucky to have a permanent spot for the rollers; so this method may not work for everyone. Basically I just hang onto the handle to get on the bike and get spinning. Once spinning it's pretty much just normal cycling balance.
The main thing is to be able to support yourself externally so that you can get clipped into the pedals, onto the saddle and build up a little speed so that you can maintain balance. I got my rollers several years ago and pretty sure that I took a tumble or two before I got the hang of it.
It's also much easier to learn if you're comfortable riding with no hands. (not that I do that on the rollers ;-))
As some of the other comments have mentioned I got started by putting the rollers in a doorway and using that to get up on the rollers.
Once I got the basic feel of it I moved the rollers to a spot where there was a full length mirror in front of me and put chairs on the side. The full length mirror was really helpful because I could look forward and see where my tires were on the rollers. I could see when I was drifting over to the side.
Another helpful tip is to put lots of pillows around for when you fall. It isn't a question of if, it is a question of when. :)