7 sneaky ways gyms trick you into buying a membership

It's happened to most all of us: We lock ourselves into a gym membership, thinking we'll go more than three times a week if we're paying a monthly fee.

After one month of treadmill struggles, we lose motivation.

This is exactly what gyms want.

While most companies would flop if no one showed up, at a gym, emptiness means success.

"Gyms want to be this product that everyone buys, but no one actually uses," Caitlin Kenney and Stacy Vanek Smith explain on NPR's Planet Money podcast, "The Planet Money Workout."

For this business model to work, gyms need to tailor to a specific customer: the person that thinks they're going to work out, but rarely will - the "casual customer."

Here are seven strategies gyms use to get you to buy that membership: