Rafflecopter is awesome for this.I have a client that does it regularly. What we do is add an option (one of the make your own entry options) that says something like "If you already own this product post a review for an entry" and other one we do is "Write a blog or forum post about the product for an entry"
I would say doing that we normally get 100 or so positive reviews on various sites and 30 or so blog posts / forum posts. We typically get about 1k people entering total. One thing we have been doing to further target is to post suggested review or forum sites to post on. I think it is borderline spammy, but I feel like it is on the safe side of the line.