I found a site that can help members who are running sweeps. The site will generate a random number for you, based on the information you give it.
http://www.random.org/nform.htmlWhen I ran a recent giveaway, all eligible people were allowed one entry. I deleted duplicate entries and ineligible participants. This left a list of all entries (35), barring entry #1 which was my explanation of the contest. I went to the random number generator and told it to give me 1 number between 2 (the first post that wasn't mine) and 35 (the total number of valid posts).
The site immediately gives you a random number in the range you name. See which post corresponds to that number, and you have your winner!
