Flatten Warped Boards

I have some old 9" oak baseboards that came with my house. I toolk them all off months ago to insulate and rewire and am now ready to put them back up. Problem is, the face side is varnished and the back is not, so over the years the back has dried out and the boards are all warped now. This isn't a problem in the center of a wall, but makes it just about impossible to make nice mitered corners. The boards are abou 3/4" thick, so just bending them straight isn't going to happen.

Some book once told me that to bend or round a moulding, i.e. something non-structural, you can cut slots about 2/3 of the way through the material from the back, in parallel with the direction in which you want to bend the wood. The material will then act as if it were only 1/3 as thick as the original, making it that much more flexible. The more slots you cut, the more even your bend will be.

I made 7 cuts along the back of the boards on my tablesaw. At that point I could bent the boards straight, but they wanted to bend into a "W", so I think for future reference there will be more closely spaced cuts nearer the edges. I am sure there would be a way to calculate the ideal spacing, but I won't even go there.

With the cuts spaced as it is I was able to clue up the corners in a decent mitre joint.