Jeff (the owner of the site) reads each and every post that goes through here, and he rates every one of them. An average regular post will just give you the +1 in your post count. An above average post with some new information will give you +2 in your post count. If you make a really good, descriptive post (doesn't even have to be long -- just informative), your count jumps up +3. I've heard of some of the really good posters going up +5, but never actually seen one of the posts.
Your post count becomes eligible to be deducted from once you reach 100 in your post count. Once that happens, if you make any retarded/stupid/bad/worthless posts, your post count will go down -1. I don't think you can ever lose more than one point from your post count, but if you keep getting -1's, eventually your count will go all the way down to 0. Once that happens, that account name gets perma-banned, and you'll have to either leave the site or create a new, different name.