1. Know your dimensions
When I first calculated the size of the box for Archon Arena, it was smaller than the box size I ultimately settled on. This meant I had written 1500x1200 in the contract when I really needed 1800x1125. That difference in aspect ratio made a big difference in the artwork. For awhile we went back and forth with me trying to get things moved farther apart before I realized he was working off of different dimensions.
2. Communicate the details
So far, for me it's been great when I'm able to ask for something very specific. For example, I wanted the warrior's hands to be farther apart so he looked more ready. I asked and quickly received. In other cases where I've been able to communicate the details, I've gotten what I was looking for. When I've had only a vague notion or been unspecific in my requests, things have taken longer or not come out as I wished.
3. Let the artist be the artist
I've previously discussed how having too specific of a vision in mind can make it difficult to get what you want. In most cases remember that the person you've hired has a lot more experience and training in art and design than you do. Give them some free reign and you're likely to be pleasantly surprised.
4. Be upfront about money
I felt like I was being overly burdensome on my artist. I didn't want him to be upset or feel ripped off by the amount of work I was asking for with revisions. I stated upfront that I understood this and would give him extra if he thought it was appropriate. (He kindly rejected my offer.) Hopefully this way we will both end up satisfied by the transaction.
For me commissioning art has been a learn-as-you-go experience. The lessons I'm getting in the school of life will surely serve me well going forward as I require more art. Hopefully they can help you too.