I usually do an exercise where I take a classic game and I attempt to improve upon its mechanics. Usually its all theory. Make a minor change here, there. Replay it mentally. Repeat ad nauseum.
Recently, I decided to create my own casual one finger mobile game as game design practice. To me, its mediocre at best. I don't know if its "good enough" to compete in the trenches with all the other starving developers. However, I have had a few close friends play and they seem to enjoy it. Dare I say... They might even buy it. So I am working on fleshing it out more in hopes that it can live up to the quality I wish to release.