Aside from just "buckling down" there isn't much else you can do.
But whenever I've stepped away from a project for too long, I always start looking through the design doc I made previously. Then, starting with a clean sheet of paper, I re-envision the better part of the game with new ideas. After that I compare the two and combine them (removing any ideas I don't really like.)
Although this hasn't helped me with finishing my games (too many revisions!) it definitely reinvigorates my enthusiasm. It's like coming back to a new project every time, which is kinda exciting.
Yeah, after thinking about it. That might not be the most helpful advice. :P