There are two places that were easiest to learn rollbacks for me. A quarter pipe, and a steep driveway. This is because they are much easier when you are rolling fast.
To do it in a QP:
1)Ride halfway up it at jogging speed so one front wheel is close to being straight up and the back is still sloping.
2)Lock both your rear and front brakes, this will hopefully preload your fork a bit.
3)Unload your fork by leaning backwards and diagnol (I do it on diagnol back and left). You can decide by what way to lean by what way you endo-whip the best. If you can do it the furthest to your left, then you lean back and left, and vide versa if you do it right.
4)Grip your left grip with all your fingers, so you can turn the whell better. This is optional but it worked for me.
5)Once you are at about 60 degrees, lock your rear brake. Continue lleaning the way you were before, which results in unweighting your back wheel, and back you spin the way you were leaning.
6)When your front wheel drops, let go of your back brake and roll away.
To do it on a drive way:
1) Roll up the drive way, a little faster then a QP. 2) Have your weight over the bars and hit your front brake doing a small endo. 3) This will give you momentum to roll backwards, and preload your fork. 4) When your front wheel hits the ground, quickly shift your weight diagnolly back (just as the QP) 5) Continue the same steps as a QP, your timing will have to be more precise however, because you will probably be going slower.
*Tip*
Don't try to ride fakie backwards to long. You can practice doing it longer when you actually know how to ride out of it. I learnt it by riding fakie for about 5 feet then doing it.
For more style learn how to do a slider (no rear brake) or a halfcab (you do more of a 180 when you roll backwards to face the right way.
Goodluck, and remember to practice, it took about 2 weeks of straight practice for me to do it, but is definatly worth it.
I will try to get a video of each step up after the long weekend.
I'm pretty sure it is the same on a bmx. One thing I forgot to say is that when you are rolling backwards, you should begin pedalling backwards as well.
roll up whatever your fakie-ing down at a moderate speed and start backpedaling once you feel the bike wanting to roll backwards. then continue backpedaling and try and keep your balance at the same time. remember that since your going backwards the bike's steering will be a little weird to get used too but you'll get the hang of it. then to turn around (the rollout part of the trick) use pedal pressure to pick the front end up off the ground a little bit and set the front wheel to whatever side you turn to best. turning around is the tricky part. its really just practice thing, it took me about 3-4 weeks of daily practice to get rollouts consistent but im sure a lot of people will get it quicker then that, so keep at it and good luck!