It's a subject I have thought about a lot and tried a few products.
Since doing a fair amount of vinyl application on car bodywork and glass I have found that Isopropyl alchohol does a very good job of degreasing, athough the surface needs to be fairly clean to begin with.
Just done a few searches and come up with these slightly relevant articles:
http://www.binswangerglass.com/clean_glass.htm
http://www.signweb.com/vinyl/cont/windowopps2b.htm
Also, this recipie comes up a few times:
1/2 cup ammonia, 1/2 cup white vinegar, 2 tablespoons cornstarch [cornflour?], and 1 bucket of warm water.
I used these keyword combinations in Google - ultra clean/streak free windows/glass
Gareth