If you are in the USA and the amounts aren't huge (see: http://www.nolo.com/legal-encyclopedia/small-claims-suits-ho...), then go to small claims court in your state to collect. But you should be sure to show (a) you were paid previously (b) what you are charging is justified, and (c)you have some proof that you tried to get the payments made (certified letter, etc.)
Small claims is probably the best option here. After winning in court, if they don't pay then the next step is to file for a lein. When they go to get a loan or sell their house, they will have to take care of the lein. So you may get paid eventually.
I took a roofer to small claims court and won. He didn't pay, but I didn't bother with the leins because I really didn't want the extra aggravation. In retrospect, I probably should have.
