It sounds to me like the reason you are slipping off of rails is a commitment issue. If you are sliding the first half fine there is no reason you shouldn't be able to slide the whole feature.
My advice would be to make sure that you are looking at the very end of the feature or landing and not down at your feet. You will go where you look. Also you may want to try hitting the feature with a little more speed so your momentum will carry you to the end of the rail without having to balance as much.
Aside from that when you are lining up to hit the feature don't worry about slipping off the side, just hit it with confidence and look towards the end of the rail and stomp the landing.
As far as spinning goes look at this thread
http://www.snowboardingforum.com/tip...ng-guides.html
and go on youtube and type in snowboard addiction and watch their videos on jumping and spinning.