You have a master list of all the members right? Send everyone a bulk email. The order in which they reply becomes their "entry #" Specify a certain date range in which to reply. Then use a random # generator (Excel has one) to pick the winner.
If you use this you will have an exact # of entries so you will only have to pick a winner once. It would be nice if all 700 members surfed the board daily/weekly but I doubt that is the case. So you could theoritcally pick 350+ #s without a winner unless somehow you know they still participate in the board.
Even if you send an email chances are the response rate will be about 25%. So 1 out of 175 are great odds.
PS Paypal idea is a good one but it excludes too many people for various reasons: no paypal acct, a dollar is a dollar and this is a give away, laziness, etc.
If everyone favors paypal I suggest that the dollars be a giveaway itself. Annother prize to sweeten the give away!
See all those stats classes in college finally came in useful.