Any time you perform a service before being paid, you extend credit. So why not create an official policy, so clients know what you expect in terms of payment?* No credit policy in place? If the policy is in effect from the start, there can be no 'misunderstandings.'
* Not using your credit policy effectively? If a client reliably abides by the basic policy, you can loosen it or offer different terms to help encourage them to take on more services.
* Extending credit to anyone? A credit check should be a major part of your pre-engagement client vetting.
* Receiving too many 'non-sufficient funds' or bad checks? Paying by check is a privilege - don't let clients take advantage of it. Your policy can spell out the conditions under which you'll accept checks.