1 part white vinegar
1 part water
Put mixture in a spray bottle and use to clean glass surfaces.
Easy-peasy! :-) At the workshop I went to last month they mentioned that you could put a drop or two of lemon essential oil in the window cleaner and it would add shine and scent. An added bonus is that when the sun hits the clean windows it will release the lemon scent into the air. Since I do not have the lemon essential oil, I haven't tried this yet, so I can't vouch for that.
But I can vouch for the cleaner! It works really well. Something I read somewhere quite a while ago (so I have no idea of the source!) is that the window cleaner you buy from the store has wax in it. So the first few times that you clean your windows with the vinegar/water, it will be a little harder to clean, but after you get rid of the wax build-up it should be easier.
A little story: about two years ago Kevin wrote on the bathroom mirror with a bar of soap. It said, "I ♥ U". Very sweet, yes? After I washed off the soap, I noticed that whenever the mirror got fogged up, I could still see his message because it didn't get fogged up where he had written. After two years, and many cleanings later, that message still showed up every time the mirror got foggy. My vinegar solution cleaned that residue right off! So I don't know whether to be glad or sad...the mirror is now "clean" (though you couldn't see the residue if the mirror wasn't fogged up) but can I no longer see his sweet message!