Aside from Avenger's drug induced approach, Donutz is on to it. Also start small in everything, rails, boxes and jumps.
I just started riding med difficulty rails this season. I started by finding the smallest rail you have ever seen and getting comfortable riding those. I then started to hit the next size up feature. This has worked well for me to build confidence and to be able to ride the feature. I am now hitting features I never thought I would.
I have found though that when I think I will be going to fast, I'm actually going to slow. "just huck it", kinda, as everyone has mentioned in other threads and my own. You do need more speed than you think you are carrying and this for me has proven true for boxes, rails and jumps.
Hope this helps and good luck in your park adventures !!!!!
-I'm Slyder and I suffer from "Gummer Syndrome"