The lessons at the Uke School kind of explain it well. That keeping a good strumming rhythm is about rewiring your brain so that you don't have to think about it. Endless repetition at different speeds helped me a lot, not even playing a song, but playing easy chord changes so I could focus my attention on keeping the strumming rhythm steady. And then I could try switching from one rhythm to another, and another, careful to keep them steady. It's like learning any other instrument, really. Repetition and concentration until things become second nature.