There is the possibility of friends voting for friends, and in large contests where there is public voting, there is still the occurence of outside non-members being recruited to sign up for a one time visit, "cast their vote" and pump up the chances for placement prior to the judging of a final "Top 25 Entries".
This is typically why contests that have a panel of judges that are the only deciding factor, is the most fair type of challenge or contest. (higher end sites with higher stake prizes)

Here in a small Forum of mostly friends, there is a trust factor. Some people may feel slighted at the results, I know I've had my moments, like "damn, my entry was better than that one".
Ultimately I've always voted for what I like, even when I was in the competiton and I knew a certain person was neck & neck with me in votes, I still gave my honest vote even if that meant giving them the edge. This would only spur me on to try harder next time and rise to the occasion of the "CHALLENGE". It's suppose to be fun creating and helping others create along the way. Learning and sharing in a friendly enviroment. There's a reason people keep coming back to this forum even after they graduate to higher end applications, "family".

Consider omitting the staff members from the competiton, submitting anonymous entries, omitting public voting, assigning  judges, prohibiting posting to the galleries before the announcement of the winners, and finally not being able to post WIPs for advice and consultation from those you want to beat in the the challenge or next challenge, or the one after that.

Other things to consider:

.....other forums at Renderosity (checked 6 or more other of the most popular forums), use their names or they post their entries to a thread.

.....there are 4 members in this forum that would be excluded from the challenges due to their ability to check member names.

.....strings of "friends" can privately IM each other in assurance of vote swapping.

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Ockham's razor- It's that simple