the only piece of advice i can offer is once youre on the rail, if it doesnt feel right dont freak out. just keep your balance steady and let yourself slide off. squirming around or shifting your weight quickly will almost always end badly
To be honest after doing it a few times the fear factor is just gone mainly because I KNOW I can do it. I am getting much better at locking in to my 50-50 down the rail and want to start to try more advanced slides on it.