First of all, I must state that I work as an artist earning a small wage for 30 hours work a week. This is my business and currently my only source of income.
Reputation - as a customer and an artist, good and bad reports of your reputation will stick to you and follow you around. Be a good customer and/or artist and your reputation will improve. Beware, a good reputation is hard to earn, but a bad one isn't.
Relationship - When you commission an artist to create work for you, then remind yourself that you are paying them to do you a service. Treat them with the respect a craftsmen deserves. Artists, customers are not your friends, even if they are in real life, customers pay your wage and you have a duty to create the work they are paying you for.
Respect - Mutual respect is tantamount to a good working relationship. As soon as respect starts to break down and the artist and customer resort to slanging matches then all sense of mature business ethic is lost.
Money - This is the usual reason that a commission breaks down. Artists, don't be afraid to ask for money but be aware that you need to meet your side of the bargain when payment is received, whether that be in one lump sum or in installments. Customers, pay what is asked and make sure you have a clear idea of what you're paying. Be sure to pay promptly and let the artist know if you have *any* problems meeting payment.
Politics - have no place here. This is a marketplace, not the playground.
Research - if you are unsure of an artist, do your research. The internet is your friend and good artists usually have a record of commissions they are working on and have worked on. Talk to the artist to find out usual time scales etc. Never be afraid to ask questions, it costs a lot less than just throwing your money at arseholes!
I hope this has helped and that I'm not preaching to the converted, but I have been astounded of late by the number of situations that could have been avoided if people realised that what they are doing is PAYING someone to MAKE SOMETHING for them, just like any normal business transaction.
Okay, rant over ^-^