Keeping wind shield free from steams

Sounds a little counter intuitive, but the answer is: Turn on your air conditioning!

If you don't like it cold, turn on the heating too. The reason that the air conditioner works is the dry air it blows onto the wind shield.

It just takes a few seconds of A/C. After that you can turn it off until steams appear again... 

1. Leave an open window

2. When the windshield starts to cover with vapor turn full heat/cold and direct it upwards.

The air conditioner is the most effective because it has an air-dryer built into the compressor (that's why you get that little puddle under your car after running it a while).  As an alternative here's an old folk remedy that's worked for me: get a fresh white onion, cut it in half, and rub the cut edge on the inside of the windows.  Smells up the car for about 5 minutes but the oils from the onion keep the windows fog free for weeks!