You'll get more entries if you change the part about using banners that don't win. I mean, it's already difficult to justify entering contests knowing that you might do alot of work and not win, but to know that your hard work will be used even if you don't win is a real slap in the face. I rarely enter contests with that rule.
You have three choices that I see, if you want to get more/better entries. You could either raise the prize offered to offset the risk of not being paid for work that may be used. You could remove the requirement of using the banners of those that don't win. Or you could offer a small prize for each banner that you choose to use that doesn't win the grand prize.
Good luck with your contest whatever you decide to do.
