Myself, like some others here, counted Brian Bilby as my smith of choice. Luckily, I have a few firearms that saw his work and count myself a lucky man....
Currently, I have projects going on with Dane Burns who I had gotten to know by way of Brian being my close friend.
Even though I have not received any work from Dane yet I have put my trust in him from the conversation's we have had, his reputation, and Brian's opinion of him as a person and his work....
I think when you decide to deal with a custom maker personalities can have a lot to do with it. You need to have a good line of communication and not be afraid to express your opinion to let the smith know what you want. Do you get along well with the smith via phone and E-mail? Are you comfortable with the way they conduct business....
You also need to do some research on your own. Do you know anyone who has had experience with said smith, what is the feedback from various people, how long have they been in business, what's their reputation in the industry.
Something else to think about. What work are they known for? If you send your 1911 to a smith who specializes in Bullseye guns don't expect to get what you're looking for if your intention is to have an IPSC/IDPA/or personal defense piece. That doesn't mean the smith can't do it, but if you aren't clear on what you want you may get a gun made to feed target ammo and built supertight when you really wanted a carry piece that would feed everything reasonably possible and go bang in a worst case scenario....
Finally, you must be willing to be patient. You are dealing with, normally, a one person shop. If they or their family is sick the factory may be down for a week, or longer depending on what's going on. I could go on longer about this, but I would recommend going to Danes website and looking at his comments on timeliness.