This might not be officially "frugal," but it addresses a problem frugalites sometimes have: finding ways to bypass the questions and strange looks.
Use a little vinegar or ammonia in water to wash windows - that's commonly known - but if you want to fool your neighbors (spouse, mother-in-law, etc), put a drop of blue food coloring in it. They'll think you're using a commercial window cleaner. If you can come up with an old spray bottle with a brand name label on it, so much the better, but maybe you can fake it somehow. All in fun, of course.