You must be careful to choose a trainer who understands what you want to accomplish and the unique qualities of the mature athlete and active ager. The trainer must understand that you are not merely a bad sequel to your 20-year-old self but are in fact unique. I recommend finding a trainer who has previously worked with masters athletes and adult onset exercisers. Interview the trainer to find out if she believes that fitness after 40 is possible. Although there are many great therapists and trainers out there, you would be surprised by how many of them buy into the notion that we should all just slow down when we reach a certain arbitrary age. Such a belief affects how they will work with you.
Just like any service, one great way to find a personal trainer is to ask your friends, family, business associates, etc., if they are working with anyone they like. You can also watch the personal trainers who work at your gym and observe how they work with people. If you see someone you like, ask to talk to him about your goals. You also want to choose a personal trainer who is certified.
If you decide to hire a personal trainer, do so for a short period of time first. Then, if you are achieving your goals with him, you can purchase additional sessions. It would be unfortunate to buy a six-month package of training sessions only to find out you don't get along with your trainer or he doesn't understand your goals.