Economists have shown that people overestimate the amount they will use an annual gym membership by an average of 70 percent [source: DellaVigna & Malmendier]. Assuming you're paying based on how much you think you'll use a gym, you're overpaying by the same 70 percent. It costs the gym very little to add a new member, and a gym membership is a long-term contract with lots of options. As with buying a new appliance, the fact that gyms run specials opens the door to exploring discounts at any time of year.
First, start by being honest with yourself. How much will you really use the gym? Then figure out how much each workout is worth to you. If you can't haggle the price of a monthly membership down to that amount, consider a pay-as-you-go card, or look into buying home gym equipment (both of which are negotiable!).