Leave the garage door open on dry days to help vent the stale air in the garage. Check humidity levels for the weather.
Run a fan in your garage. This can be done in conjunction with leaving the door open, but is also good on days where the air outside is moist. The more the air circulates, the less likely moisture is to build up.
Run a dehumidifier in your garage. Dehumidifiers usually list how many square feet they are designed for, but if moisture is a serious problem, get one size larger.
Install a wall vapor barrier along your garage walls. Vapor barriers are ideally installed underneath the wall's finish, but can be installed on finished walls with an added risk of damaging the barrier.
To install a vapor barrier, roll sheets of the barrier in a continuous line up and down the wall. At the bottom, run the barrier a couple inches below the footing, or along the floor of the garage if you don't want to go underneath. Staple or nail each section of the barrier in place. Each strip should be installed side-by-side to another strip, and sealed with seam tape.