Every dice site has tried a wagering contest (which really is just a reward to high rollers) so I would avoid that as it usually doesn't bring the desired traffic sites want right away. You could run a contest where users earn tickets for each 100$ wagered or whatever amount you wanted to make it. Users would then be entered into a random drawing with xx winners for xxx prize amount. It's not an original idea because Stake.com does a similar contest, but they seem to get a decent number of participants weekly from it.