Dear Dave:
--It occurred to me that, possibly, you could, multiply the each persons entry by his/her number of posts, and put those in the selection file. I.E. if Joe has 1400 posts then he has 1400 entries. If there are less than 5 posts, multiply by zero. Then you would select at random as previously. This would still give all legal entrants a chance, while giving you major contributors a leg up.
--Are you really paying the shipping all the way to East Wazoo?
"I sell here, Sir, what all the world desires to have—POWER."
James Watt 1736 1819
Best Regards
Clear Ether