We hold competitions all the time. The most popular forum based competition is RPS, Rock Paper Scissors
Its pretty easy, 32 contestants join and are paired off in groups of two, then proceed to send PM's to a mod, the mod keeps a track of these using a post in the competition thread. Competitors know they aren't lying because both votes are logged. Once both have been sent they are logged (and any foul play is easily found out)
This keeps going in a pyramid like structure until only one person remains. We did however have a lot of problems with inactive members.
For any further details feel free to PM me at BakaBT.me as I rarely use these forums
So what do you think? not a bad idea? We used to hand out prizes too, considering this is a torrent site, maybe you could have prizes like one week freeleech for the winner, or maybe a boost to their ratio. I don't know, I'm just making suggestions.