Happy to share experiences:
I am doing an IM70.3 next year with no triathlon background, so needed someone good to help me go from weekend cyclist to someone able to swim, do a hard weekend bike ride and then run a half marathon. I realised that choosing the right coach would help me more than cosing the best bike, so decided to invest the money in the coach and training rather than just buying a tri bike and visiting the local pool a few times on my own.
My approach had a few steps:
1. Write down my needs:
- Not too expensive
- No formulaic X phone calls per month limits
- Not too many athletes
- Real tri experience, with some demonstration of solid personal / coached athlete results
- Some coaching experience, ideally a formal qualitifcation of some sort
- Returns my initial phone call / emails reasonably promptly
- Coaching can be German (I live in Switzerland) but some English would make things easier
- Able to tell me clearly how he / she would work with me
- Able to explain his training philosophy
- Able to diagnose my likely strengths / weaknesses based on some basic history
- Focus on measurement so I can see progress, ideally using trainingpeaks or similar
- Ideally local. For me this is important as I'm quite new to swimming and running, so have a lot to learn. If you're already an experienced cyclist (10+ years, club participation, many types of racing) then you probably have the techniques you need. If not then I would not underestimate this.
- Group training - Indifferent. Long distance tri is an individual sport, and I need to fit training around work and family so I will rarely manage to join a group. But for road racing I'd say this is very important as "chain gangs" and such like are a great way to build technique and cycling group riding experience
2. Make a long-list
I reached out to a few people and also emailed coaches from local tri clubs. Things may be different where you live, but the people I reached out to either did not respond or were not very helpful, probably because I don't know many people who do triathlons. Most effective route was to look at the Trainingpeaks list of coaches. I researched a few of those who were focused on Triathlon, either nearby or from my home country. I quickly ruled out "celebrity" coaches who charge $$$ for a small slice of their time.
3. Narrow down the list
I sent out a short email to about 10 coaches describing my goal of doing an IM70.3, my sports background, what I was looking for in a coach and asking whether they could help. About half did not write back, 3 wrote back quickly and 2 were slow, without any excuse. The slow responders and non-responders were elminated. I was surprised to find that there is little or no correlation between the quality of the coach's website and their responsiveness.
4. Finalise your selection
I swapped emails with 2 coaches, then did a 30 minute call with each one and thought both would be able to help me. However, Coach A was consistently easier to deal with, e.g. said he would call at 4pm, called at exactly 4pm or emailed me that he would be a few minutes late. Coach B was also vague about pricing. Eventually I found that there would be a lot of add-ons, and that his fee would be double Coach A. So then I met Coach A in person and after that we were both fully comfortable to work together.