well depends .. if you have money, you can take the contest to a whole different level
you could award a xbox game, case, or something that would look cool
and since it would be free and all you need to do is make a signature
then people will just post and try as hard as they can
but if money is not a option
maybe special VIP features..
stuff where they can get special downloads, exclusive tutorials, i don't know but you catch my drift..