The biggest and best piece of advice I could pass all is Know the artist. Study their past works, look at how they draw things, look at the content they enjoy. Look at the difference between past commissioned work and their own personal work. Why would you do this? Every artist has their own tastes, they enjoy some things more than others. For example, I wouldn't approach Xian Jaguar for Tentacle rape. As hot as you might think it'd be, it's just not going to come out looking good. (Or in her case, I doubt at all) ;) Everyone has thier own style, make sure it's what you're looking for.
You want to know what *your* character will look like? Look to see if that artist has drawn your species before, or the closest match. That will be a decent preview of the basic outline for how you're going to be drawn. If you don't like it, then maybe that artist isn't for you. Be a smart shopper. The artist is still going to do what you paid them for, but you should at least attempt to prepare yourself.
Finally, do a background check. Do they have an already long list of people waiting for art? Expect to get in the back of that line. Do they have a history of being uncooperative, ignoring you and drawing what they want? Perhaps you shouldn't get involved. A lot of the commissions I've gotten have been because friends have had good experiences with them, just as I have recommended things over to friends.
Very similar to the first section, but this deserves it's own. I've gotten a lot of very nice pics done in the past, not so much because *I* have good ideas, but because the artists do. They're the creative ones. So at the most I'll provide a description of the characters, one or two visual references and a basic idea of the scene. From there I like to see the artist's interpretation of what I've given them. I actually encourage them to be adlib things, and add their own ideas in both the character design and posing, etc. Now I don't expect this to be for everyone, but the factor in this equation is that it allows the artist a great amount of creative breathing room. Usually this benefits you directly.
I've seen pics where the artists were annoyed at the commissioner, or where there was just too much in the way of 'knitty gritty' detail work. You'll never get the piece you're looking for that way. The artist will almost never be able to draw what *you* see inside your head, though some may get close, if you're lucky.
This is a big one for me. I expect communication from the artists I commission, and I give the courtesy of communicating back. I ask them what they think after I pass along ideas. I get feedback from them and feel them out. Maybe you have an idea in your head, and maybe they have a better one. Perhaps your pose is anatomically impossible, and you've overlooked those minor details. Keep an open ear and an open mind and make sure you're both on the same page.
This is one of the most delicate areas. From my own personal experience, this can quickly sour a decent business relationship as well as spill over to the quality of the piece. The most I can do is advise that you treat it as a business transaction. You are hiring someone to do work for you, as the customer you must be clear and prompt in your transactions. I would never suggest that you pay someone upfront for work where you have not even seen a sketch. A promise over IM, Email or Message board isn't worth much. I would suggest that you turn over at least 50% of the total agreed payment once you've seen and agreed upon a sketch. Then depending on what you and the artist work out, the final portion upon completion or near completion. The artist may choose to show you a low res version of the final product until you turn over payment, or maybe something else. Be fair, be honest and be quick with how you do business with someone, I've had a few bad run-ins with some very talented folks, and I'll tell you now, no matter how good someone might be at what they do, it just isn't worth it if they can't do proper business.
This also works on the flipside. Artists talk, I've seen them do it. If you treat one artist poorly or rip them off, you'll soon find it hard to get work from any future artists. And if they happen to do work above and beyond what you expected, a little tip goes a loooooong way. (You might even find an extra surprise too, lagniappe)
As I stated earlier, I've had a lot of work done for me, it's a hobby, I enjoy
That's about it. If any of you feel you have something you could add, I'd love to hear it.