Be very cautious about giving money for 'emergency' comissions. Once the money is in hand any incentive for finishing the picture is gone.
Be very cautious about comissioning people who have no track record
Do not pay full up front unless you really, really trust the artist. Nothing kills the desire to finish something like already having spent all the payment and knowing you're not going to get any more money, no matter how long it takes to do it.
Agree up front approximately how long the picture will take. Reserve the right to cancel if they take longer than the agreed time.
Do not order endless, nit-picky changes
If the agreed-on time hasn't expired yet, do not nag. It will just get the artist pissed off.
Do not take more comissions than you can handle. If you're backed up more than a few months, stop taking commissions until you've cleared your backlog. Do not take comissions if you know you won't have time to do them.
This bears repeating: DO NOT TAKE COMISSIONS IF YOU KNOW YOU WON'T HAVE TIME TO DO THEM. Don't think--"Oh, I'll have time later in the year." You won't. You'll just end up owing a pissed-off customer money.
Do not send the customer a nice scan of the finished peice unless they've fully paid, or you really, really trust them.
When the sketch is approved and you're ready to start on the final picture, make sure to let the customer know that no more changes can or will be made.
Agree ahead of time that if the customer reneges on payment, you have the option to sell the picture to recoup your costs.
These are hard-won trueisms earned by being on both sides of the fence--trying to commission people and being comissioned. In all my years of doing this I've had only one person renege on payment (he vanished and never resurfaced) but I've been ripped-off, lied to and jerked around by a good half-dozen artists whom I've tried to comission. I'm not going to make any of their names public unless I see them soliciting for more comissions, or if someone asks for information on any of them (like if someone posts asking if "X" is a good artist to work with)