Financial incentive would be definitely abused (look at sig spammers doing it for pennies) but something like this - why not:
a special rank (similar to Donator & VIP, e.g. contributor) after X amount of posts and also somewhere later maybe throw in a one time user-name change.
If reporting statistics (number of good reports, accuracy) are publicly displayed, that could also act as an incentive.
All you need are bragging rights for some people to get motivated.