If you don't have time to read the whole thing, or want to get a taste of the contents, here is a summary of the important points.
The Project Triangle: There are three variables for most any type of project: fast, cheap, and good. You can (at most) choose two of the three. Don't expect to find a great artist who is inexpensive and able to work fast.
Realistic Expectations: Remember that artists want to create. Try to see their work for you just as you looked at their other work, with few up front expectations. "Once you wed your mind to an image beforehand, you’re more likely to be disappointed."
Dollars and Sense: Going along with realistic expectations, keep in mind the salary an artist has to make to survive. Factor in how long your piece will take to create, and you can calculate a fair price.
Know Your Rights: This section explains the aspects of intellectual property licensing. A key takeaway is that you can potentially save money by less restrictively limiting the timeframe, geography, and/or types of media in which your commissioned art can be sold to others.
Vintage- Not Second-Hand: With all the art that has already been created, consider licensing existing art instead of having new pieces commissioned. This can save you a lot of money.
Actually Commissioning Art: This section summarizes much of the information about and adds bits of added advise.
A Finished Project: The author suggests similarly hiring a Graphic Designer to ensure that the great artwork you've acquired is set in equally great design.
If this summary has whet your appetite for more details, go ahead and download the file linked above. It's a good read and extremely helpful for the new indy publisher. As it says toward the end, "Congratulations! You officially know more now than every small-press client I’ve worked with, coming into their project." So check it out, and you will be educated too.