SFB's advise is good and if i can offer up anything else, its having confidence and being balanced and ready the second your wheels hit. like with a manual i feel like you have a little time to find your balance after you get on the obstacle, nose manuals you have to be ready for it before you even get going. be light getting into them too. if you stomp your board down, its going to be hard to keep it going.
keep at it and you'll get it. if i can do it, anyone can and its one of my favorite tricks.