So far, I have only had 2 (small) clients! I invoiced the Realtor net 30 upon completion of his flyer. The horse rescue was initially billed bi-weekly while I set up their books and organized/back entered their 2006 and 2007 records. They will be billed quarterly in the future as maintaining their records is only 15 minutes/month!
The job will determine the billing. I offer pre-paid monthly blocks of time which will, of course, be billed in advance on the 1st of the month. Otherwise, for 1-time or small projects, billing will be upon completion of the project. For on-going projects, billing will be the 1st of each month and due on the 15th.
I request that all jobs (even the little guys!) sign a contract before I begin work, which gives me the right to charge interest for any past-due items and they agree to pay any collection costs required. This gives me a little more power should I have to go to collections or small claims to get paid.