Me personally as a commissioner:
I open in January for the year. From there, people email me. I receive over a hundred emails a month (and honestly, I'm probably low-balling that number). I require a downpayment to secure a slot for commissioning. I then remove that calendar slot because it is taken for a commission. I'm able to track what commissions are set and ready to go and ones that were just an inquiry.
Because I'm a desired commissioner (I suppose), my calendar/slots will fill up fast. Right now, I'm booked until October. Yes, I have the downpayments and yes I have a schedule of when to start/finish commissions. The money stays in my PP account until it is needed. Usually materials, especially wigs, are bought on-line.
I know there are some jerks out there, but then there is someone like myself with good ratings and a system that works for myself and the costumer.