Oh man - good luck. Probably best method is reference. I have gone through 5 coaches and couldn't really refer a single one. For the most part it has been disappointing. I am close to the point that a good book is probably a better way to go.
Define what you want in a coach and how you want him to work with you. So far every coach I have worked with was unwilling to meet me halfway to what I thought I wanted to work on or how I wanted it delivered.
Coach A - Face to face with guy who got me started. This is when I was first starting out with Hold'em. Not a "real" coach and I didn't pay for it. I out grew him in about 6 months because I was way more motivated to learn than he was. We are poker buddies now and I end up doing more coaching than he.
Coach B - Emails. Sent him tournament hand histories and he rightly discovered I was being a wimp and choosing to limp when I should be shoving. But once he discovered that he refused to look for any more leaks since this was such a glaring problem. Also never expressed why or took time to teach - would just flat out say "don't do that - you're being a donk". End of lesson in 2 minutes for $50.
Coach C - Via Skype. He really wasn't a coach so much as a teacher. Had no interest in anyone talking but himself. I think he got more out of it than I did but cause it allowed him to articulate and regurgitate what he had learned - but if I said anything he would just wait impatiently and blow me off so he could continue with his lesson plan.
Coach D - Face to face. Full on poker pro w/WSOP cashes (and more). Total opposite of C. No plan at all. More just a sounding board and I pay for the privilege of his wisdom. Still working with him but I have to figure out how to come up with my plan so I can get the most out of it. Asked him for a little more structure or to at least know what we were going to talk about next and got the old "this is what I do and how I do it - deal with it". He basically confessed he had no idea what he was going to talk about with me until we got together and sat down. I got with him based on another player I ran into who had some good game and definitely talked the talk. I just flat out asked him if he had been coached.
Coach E - Mostly face to face and phone. Resident Russian grinder/pro. Pretty dogmatic black and white in his approach. More of a poker buddy now, but I tell you he did something that was priceless. He sat down at my table (across from me live) and sweated me for about 4 hours. Later he listed a whole bunch of my tells and how I misplayed some hands and some general strategy that helped immensely.
Oh there was a one time session with Dennis Phillips that was just gold. He taught, he coached, he was awesome.
In the end there are probably more so called coaches that suck than are good at it. I have to think if a so called coach doesn't take the time to even see or hear where the state of your game is at then he is all about himself and you are just a revenue source.
Here's a list to start with -
http://www.notedpokerauthority.com/poker-coach-directory