All people whom you are following on Twitter, are following you back? I am sue you will say I don't know ? Here is an online tool called Huitter which can mass unfollow people who are not following you back, in a single click of a button. Its one of my favorite tool as it is very fast and easy to use.
How to use it :
1) First visit Huitter.
2) Now log in with your Twitter user id and password. Well, according to me its safe to enter password in this site but if you have any doubt that they store your password please do not try this service at all.
3) After entering username and password they will automatically remove all the people who are not following you.
Also read : Create your own Twitter like service