What can I do to keep the car windshield from fogging up?

My heater core went out and need to get by somehow while I save for this really expensive repair job. The small 12v ceramic heaters are useless, I've been using a portable propane heater, helps a little if you have someone hold it while you drive, doesn't help when driving alone 'cause it shuts off with the car moving. Not to mention I melted the corner of my shifter holding it to close. I was trying to find an auxilary heater for an old VW beetle, can't find one? Would keeping the inside cold help more than heating it? Any suggestions?