Potato Juice.
Yes, slice a raw potato and rub the open face on the glass, then wipe up.
A guy who made his living alternately between beeing a chef and a scuba diver told me this. It worked on ski goggles when I was a kid, can't say I've tried it on a windsheild.
Carrying a squeegy on your lap works too, much easier than loosening the belts & using the back of your hand (probably safer too).
Matt