shokhead

Bar Keepers! Won't hurt glass. Will take anything that shouldn't be on there off.

Steppenaudi

Super-clean your windshield (look for videos on YouTube):
1) Use Invisible Glass cleaner & microfiber towel to clean glass
2) use clay bar with appropriate lubricating fluid (one combo is ) I use the clay bar from Griot Garage plus their Speed Shine liquid.
3) Re-clean with Invisible Glass & microfiber towel.

You can use 0000 steel wool to scrub off crud, but I've found clay bar to work better.

choppstixxx

I will also say especially on brand new vehicles there is always a film all over the windows. I use a razor blade and scrape em, you'd be surprised at what comes off.