I'm a moderator at a small phpBB forum and we recently held a writing contest. The winners to be decided by a standard phpBB poll.
Our goal was to determine the best story, based on the judgement of our diligent members.
The various authors, of course, wanted to "win" -- sometimes no matter the method.
We soon had sock-puppets and friends and family of some authors signing up just to vote and then disappearing back into the ether. When we required at least 1 post to vote, they'd post something generic, vote, and leave.
Then again, some "family" (2 different email accounts, posting from the same residential IP address) did take the time to critique the stories and/or otherwise contribute.
How can we detect sock puppets, and how can we differentiate them from family or coworkers behind the same firewall?
How can we prevent friends from signing up and just voting for their friend's story without evaluating other stories?