As a gym manager, here's what I have found to work:
Programs: Each trainer has their own skill set and expertise. Use this to your advantage. As long as they have good skills and a sound science base, your facility will be able to reach a broad range of clients.
Pay: no independant contractors! Each trainer works 10-20hrs per week on the floor at a minimum pay (you know, answering questions, building rapport, giving tours etc) and a higher rate for seeing clients (about 60%)
Hiring: Interview...gives you an idea of personality. Case Study: gives chance to test knowledge. Mock PT session: to assess client interactions.
Ongoing evaluations: I video actual sessions twice per year, and review with each trainer. Those who improve get raises, those who don't are shown the door.
Provide perks: I sent all trainers to 2 conferences per year. Provide in house continuing ed too.
Results: High trainer retention = high client retention = increased profits for facility.
hope this helps