Apart from having a warm car to start with... heat is your friend but I assume the fog is from a cold start in the morning?

The fastest is the A/C. Also, make sure you are NOT recycling the air. You want to remove the moisture in the air.

There are some anti-fogging solutions out there, you apply them on your windshield but I think they leave more of a mess then help. A clean window (use stoner's) is also your friend.