I noticed that the board I planed yesterday wasn't exactly 'flat' even though I hand planed one side. It still had some rockin' and rollin' going on. I thought today about making a planer sled, which I've read about online. Some of them seemed incredibly complicated, like the one from FWW. I didn't want to spend an entire weekend making a jig (even if it would prove to be useful). I found another one that looked more my speed. With that, I spent a few minutes cutting up a piece of MDF and screwed a cleat on the end.
After locating the points at which the board 'bowed' up I put shims under these locations. A couple of passes later, voila, a flat board on one side. Now I just have to put the other board on it to get it flat as well.