I get nervous on rails too. It's gotta look just right or I won't try it.
Start out doing 50-50s on big wide boxes. Don't think about it too much, just look at be beginning, hop up onto it, look at the end, ride there, hop off and ride away.
Here is an example of how to do it. Nice and easy. If I can do it, you can too.
http://renegadesrendezvous.com/video/TB_2006-12-23.wmv
Stay away from the booze until after you have done it. Consider it a reward.
On this day, no booze was consumed until after filming.
Have fun!