I used a degreaser to clear years of oily residue from the inside of my windshield (residue that Windex wasn't cleaning up). The areas that I missed fog up much quicker than the cleaned areas.
Speaking of which, now that fall's here, I think it's time to degrease my windows again.
Last time I used a biodegradable bike chain degreaser. This time I'm going to try my new favourite cleaner, LA's Totally Awesome Orange Degreaser. It's $1.25 at the dollar store, and it is unbelievably effective. Years of discoloration in the grout in my kitchen/bathroom/entry tiles? Gone. Apparently it's got toxic stuff in it, so gloves are advisable.