Agree with rogerd. We have long considered using incentives to boost user participation on the site, but it is a slippery road. Once you go down that road, incentives might be expected by your members all the time. And if you stop, they could feel slighted and stop using the site.
I think some incentives/promotions are fine, but don't make it a regular habit unless you plan on doing it for the long haul.
Just my take! Others may have a different experience!