As someone who finally got this last season, I think I can speak to this. In my case, the biggest problem I had was that I was keeping my legs too straight. Because of that, I didn't "feel" the bump quickly enough and I'd end up launching off the first bump, knuckling on the second, and it was all downhill from there (and not in a good way).
Bend your knees more and relax and visualize your upper body keeping a level line while the board goes up and down over the terrain. It takes a bit of practice (not a lot-- really) but once you're able to suck up on the hills and push down on the valleys you'll find that your riding in general will be much improved.
Coffee is your friend. Everyone else is an acquaintance.