It’s clear from how you are asking that you realize that tact and diplomacy are paramount in your communication with those who won’t pay their bills in a reasonable amount of time.  

Without having to add a late payment fee to your bills, you may want to try contacting them and using phrasing similar to the following:

“I understand that in your busy schedule, you may have neglected your financial obligation to me for services that I performed for you.  Please understand that I have financial commitments and I need you to honor our business relationship and remit your payment as soon as possible.”

This may not work, but at least you’ve been professional without being aggressive.