It's simple. Make the best game.
Judging criteria include the basics (is the game winnable? is it longer than a few minutes?), originality, progression (does it get progressively harder?), appearance, easy to learn (can a new player understand what they have to do to win?) and likeability (how many "likes" does the game receive?).
All original games created between 09/27/08 and 10/10/08 will be automatically entered.
For contest rules, more detailed judging criteria and example games, please see the contest banner on the front page of PlayCrafter.com.