How to prevent ballot-box stuffing?

Should have thought this one through a little better...

[dataguy]

This month, on one of my web sites, I decided to have a "Meaning of Life" contest where people could enter their thoughts on the meaning of life and then vote on the one that they thought was the best. As an enticement, I offered a prize for the top 5 entries that recieved the most votes.

I guess it was a little niave of me, but I simply recorded the user agent and IP of the votes and thought I'd tally them up at the end of the month. The average entry received about .25 votes, and the good ones have 3-5, but there are quite a few lousy entries that have hundreds of votes, many coming from different IP's and User Agents.... it's obvious someone has been stuffing the ballot box, but I don't have any way to prove it outside of the IP/UA data that I've collected.

The contest was very successful and I want to have a new one for December, but I'm wondering if there's anything I can do that will help stem the massive vote fraud. I'm already not displaying the vote count until the end of the month... any suggestions would be appreciated!

[MichaelBluejay]

Well, it's too late for this one, but use a combination of IP/User Agent and Cookies. Record a cookie on the voter's computer when they vote, and when the vote is sent to you, put their cookie info in a database along with the IP and User Agent.