I've been in this situation before. It seems most of us tend to be on the 'nicer' side and even when a client doesn't pay we often continue to do work for them. Over the years I've come to learn that you cannot survive in business this way. When you do work for a client, you must get paid for it...otherwise you won't be able to continue to do work for people.
Whenever I submit a quote I always state the payment terms clearly AND the penalties for late payments.
If a client is more than 15 days late on a payment, I send them an invoice for the amount plus the penalty. After 60 days, I will submit a claim to small claims court....but it's never gotten this far.
Stay professional and clients will pay you on time. If they know you are a one man team...they may figure they can simply not pay you and get away with it.
Recently I've also stated that if the entire amount is not collected within the due date all content delivered to the client becomes my property and the client must destroy any copies. The more you get in writing, the easier it is for you to make sure you get paid for the work you do.
