try doing a 180 first , spot the rail and give an extra 90 to your lower body. keep looking up the rail .
the reason why you guys are sliding of the rail is cause first you are giving your self too hard of a spin for the 270 on and you are not patient enough and want to see the end of the rail too soon. and thats normal cause its a scary trick at first. just try to be more and more patient and you will feel the trick coming .