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I like @riskdebenair's idea; but I think it would also work on one post, with a 12 hour window to write, and a 12 hour window to vote. People would start remembering when to vote. And if you wanted to nudge them into returning and voting, you could comment on their thread to that effect. You could also have a rule that to win the submission you have to vote for at least one submission that is not yours (you can also vote on more, and on yours in addition).