I used Bon Ami in bar form a long time ago. Here's how I was taught to use it: Rub the bar with a damp cloth, put the Bon Ami on the windshield with a circular motion, let it dry to a haze, then buff it out.
I used it both inside and out. It took the smoke film (my dad was a heavy smoker) off of the inside of the windows beautifully.
Ditto to what Trav said - you want the 1886 formula, not the regular one. Learned that the hard way a few years ago.
And, thanks, skyactiv! Just ordered a few cans to pick up at a nearby Ace. $1.59 each compared to $8 or so on Amazon.