Any artist considering taking commissions as a steady source of income (a.k.a. those who keep plenty busy and always have things lined up) would, in my personal opinion, benefit greatly by taking a leaf out of  flaredra's book. What am I talking about? Trello! Or something very much like it. Maintaining an organized, regularly updated queue helps you AND your commissioners (a.k.a. me) keep up to date on what progress is being made. People who put down a considerable amount of money on the promise that you'll deliver will get antsy if they feel like they're out of the loop. It isn't a good feeling. And if it goes on long enough, they might just decide to look for somebody else the next time they have an idea. It's just as much to your benefit as it is to your clients' benefit. Think about it :)

Best wishes,
Ani