I found that having a hard deadline is a great way to be motivated. I once built a game from the ground up in two weeks. It was lacking a lot of polish, but it was a working game. I also entered another competition and was able to build a comeplete game in around two month and was very happy with the result.

I also find that keeping the game simple is also very important. It is very easy to add too many features and end up with a huge project that never gets finished.