At one time I used a program that would unfollow those who did not follow back. I don't recall the name of the program. Since then, I have heard that those programs can run afoul of twitter's rules. How are folks getting rid of non-followers now? It has been a while since I promoted via twitter.
A free site I use every day is Tweepi. You should use the last page of the flush tool to unfollow those you have been following for the longest who aren't following back.
You can try Twitter Karma, it's passive and doesn't auto-delete non-followers. It shows you who is or isn't following you and you can choose what to do from there.
Another one you could use is justunfollow. You can go in and weed out the ones you don't want and follow the ones that interest you.
I use this. It takes a little more time, but I prefer to manually unfollow people rather than mass. This is a middle ground.