You raise an important issue for small businesses, which is having a system to collect payment from clients.

Once you finish performing your services and remain unpaid, your leverage for getting paid drops. You'll want to follow up with the business (in writing) at various intervals. For example, sending your invoice immediately, now that the work is done; sending a followup letter or e-mail seven to 14 days later (if the invoice is still unpaid); yet another followup another sever to 14 days later.

Depending on how much is at stake, you may want to consider bringing the matter to small claims court, but that's a last resort and not something you want to threaten in the first missive to the company.

Also take a look at your contract. If it states that you will be "paid within 30 days of receipt of your invoice," and you haven't sent an invoice, you can hardly be upset that you haven't been paid on the spot.