Make use of the sanitizing gel in most gyms and don't touch your face between sets. Most colds are passed on through hand-to-nose contact.
Avoid touching the spout when drinking from the water fountain.
Bring your own yoga or Pilates mat. Make sure you wash it regularly.
Wipe down all equipment before and after use. Stash sanitizing wipes in your gym bag.
Always wipe down the handles on the elliptical with a sanitizing handwipe. Not only can this decrease your chance of transferring a variety of bacteria, but you'll be able to burn more calories precisely because you'll use the handles and engage your upper body muscles during cardio.
Wash your weight-lifting gloves routinely with detergent in hot water (and don't touch your face when wearing gloves). Studies have shown that staph bacteria can bind to polyester, the material of choice for many weight-lifting gloves.
Always place a towel on any equipment that you are going to sit on.
Wear flip-flops in the locker room. The heat and moisture from showers, saunas and hot tubs creates a breeding ground for mold, bacteria and germs.
Cover up any open wounds with antibiotic ointment and a bandage before heading to the gym. Germs can enter the body through uncovered abrasions and lead to infections.
Do not share towels. It's one of the fastest ways to spread bacteria.
Thoroughly wash your hands for at least 30 seconds after each workout.
Once home, always make sure to get out of your sweaty gear as soon as possible.