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.
Vanity-type incentives are likely the best ones to provoke people to do something. However, I wouldn't necessarily think of a rank as that would potentially add some trust issue-type problems, of people thinking that they're more reliable than others. Would an icon work? (Though it would have to be a lot of reports, and perhaps with a minimum % accuracy)