If you want to stay in shape, you need to find an exercise you enjoy and break a sweat and sustain that state for 40-60 minutes, 5 or 6 days a week. Period. What's a personal trainer have to do with it? For the most part, a personal trainer is a waste of money, a crutch, and a way for people to delude themselves into being on a "training program." I see two exceptions to my You Don't Need a Personal Trainer Rule:
Exception #1: I suppose if you're completely at a loss as to what to do with your training routine, a personal trainer can be a good thing as a temporary step, a sort of training wheels before you ride a two-wheel bicycle on your own.
Exception #2: If you're a competitive athlete or an actor and you hire a personal trainer to push yourself beyond the usual limit, but this scenario doesn't apply to most of us.
A good rule for exercise is that you should be comfortable enough while you're exercising to have a casual conversation with another person and not be fighting for breath. The second rule is that you should enjoy what you're doing, be it tennis, swimming, walking on a treadmill, cycling, whatever. This way you'll be consistent. I've never known anyone who hates exercise or hates the inconvenience of going to the gym who stays with a meaningful exercise program over the long-term.
I'm fortunate I guess that I was born with the workout gene. Since 13 years of age, I have been addicted to working out. I have to break a sweat for an hour a day or my day doesn't feel complete. It used to be Olympic Weightlifting, then bodybuilding. Now it's power yoga.
To me, hiring a personal trainer is a waste of money and an abnegation of personal responsibility.
I also see a deeper attraction to hiring a personal trainer that is rooted in childish, naive fantasy: Some blob who eats pizza and all-you-can-eat buffet every day sees a photograph of some beautiful model or ripped dude with six-packs and this blob hires a personal trainer so he or she can look like the image in the photograph. You can't go from a shapeless mass of lard to a billboard model in six weeks. But there is this American Consumer Fantasy, rooted in our collective infantile brain, that we can throw money at any problem or goal and instantly get the miraculous results we demand.
The More Adult Approach: Break a sweat for an hour a day, eat real food (as clearly explained in Michael Pollan's In Defense of Food: An Eater's Manifesto), and don't deny yourself your "pleasure foods." Simply eat them in sparingly. If you try to completely stop eating ice cream, for example, you'll obsess over it and actually overeat it. If you exercise for an hour a day and eat real food in moderation, you may not look like a billboard model, but you'll look better than the sedate blob who indiscriminately gorges on processed foods and you'll have your pride and your health back. What more do you want?