I am using Why Do Work Wordpress Plugin and works fine and really having very good features. Click here to get more information about this plugin. It will lead you to offical site of that plugin. The best part of this plugin is that you can change ad style is your post is "X" days old.
I'm with most of the people here, using All in One Adsense and YPN http://wordpress.org/extend/plugins/all-in-one-adsense-and-ypn/