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.