Step 1: Buy a cool new pair of sneakers (like these Brooks running shoes I got at Nordstrom Rack).
Step 2: Do not allow yourself to wear them anywhere but the gym.
Step 3: Seriously, nowhere but the gym. Not around the house, not on that quick run to the market, not out on a casual Friday night even though they would look pretty sweet with that T-shirt you have on.
Step 4: But—NO. No buts. You want to wear them, you have to go to the gym and earn them.
Step 5: Keep this edict in effect for two months. By then their novelty will have worn off, and any motivational power they had will have been drained.
Step 6: Start again at Step 1. (At this point, I give my least-favorite pair of sneakers away to charity, so my closet doesn’t become a shoe graveyard.)