If you want an example of what not to do, here it is: don't pay people to post positive reviews of your product, whether they own it or not. For extra stupid points, don't pay people to mark as "not helpful" the bad reviews of your product. Unless, of course, you want to end up looking very, very stupid.
Update: Mathew Ingram explains just how this kind of thing backfires:
As I've often said when I talk to groups of marketing people about social media, this kind of strategy -- or even Wal-Mart's disastrous motor-home adventure -- seem like a great idea, right up until someone finds out about it and blows the whistle (and surely by now everyone knows that's going to happen eventually, the Internet being what it is). And when that happens, you will not only lose whatever goodwill you thought you were buying with your 65-cent reviews, but you will lose a bunch more besides. You will wind up in a hole, since people will now believe that even things you didn't pay for were either paid for or fraudulent in some way.