Put it in writing: If you commission artwork for your book, you should: 1) do so on a "work for hire" basis and 2) put in writing. In response to a question from a customer, I did some research and have decided that this advice is a little too general.
A work-for-hire arrangement, where your intent is to buy the artwork outright and upfront, is simpler in the long run than royalty, shared proceeds, or partnership deals. This is why I recommend it in most cases. Because of its simplicity--the artist does the work and gets paid--I haven't heard any stories about authors and illustrators or photographers coming to blows over the work-for-hire results. However, because copyright law favors the creator of a work, you are at some risk if you don't dot your i's and cross your t's. Apparently, to be safe, you should agree in writing with the artist that you will own all rights to the commissioned artwork once you pay for it. Moreover, you should make this agreement before the artist actually begins the work.
Although I have favored an informal memorandum of agreement, you would be safer with a more formal agreement with, sigh, the requisite legal language. I did find a Work for Hire Agreement template--it's only one-page, which qualifies in the legal world as a simple form--on the website of Lloyd J. Jassin, a New York attorney specializing in publishing law.
Note that this form would set you up to own all of the rights of the artwork. In my world, I want to be friendly with my vendors. Thus, I wouldn't want to restrict the artist from using the work in a way that might benefit me or at least do me no harm. For example, I certainly would allow the artist to show off the work as a sample in a portfolio, brochure, or website. I might even allow the artist to make prints of the work for sale, perhaps for no more than credit on the back of the print. However, it appears that those secondary arrangements are best made after you've safely acquired all the rights to the artwork. The Publishing Pro.