If you are new to exercise, then you definitely need a personal trainer to get you on the right track. But even if you have been at it for years you still need to consult a trainer periodically. When we do something day in and day out we may develop bad habits, let our form become sloppy or just get lazy and not push ourselves. If you consult a personal trainer two or three times a year they can help get you back on track and suggest the most up to date routines. This will allow you to continue gaining ground and keep your workouts fun. I think twice a year is the minimum because we tend to change routines with the seasons. When was the last time you saw a personal trainer?