I know of window cleaners who have done this, and have got some work from it, especially residential.
The problem you'll have is that you will get calls from all over town, and really that's not ideal. When building a round, you need houses that are as close together as possible, as of course you don't get paid for travelling and set-up time.
The best way to get this is (you know what I'm going to say!) CANVASSING.
Most people hate doing it, but choose the right area, be friendly and dress smartly, and you'll probably be suprised just how quickly you get work.
You'll develop an eye for houses that need a window cleaner, pay special attention to the top 4 or 5 inches of glass on the windows. This is where dirt will collect, and it takes about 4 or 5 months to build up. So, if you notice it, you can be almost certian that they dont have a window cleaner, try knocking.
I know the thought of it makes us cringe a little bit, but canvassing really is the best way in my opinion.
Failing that, if you prefer to drop leaflets than canvass, make a methodical and planned strategy about it, and you could even follow it up with canvassing afterward.
I know a few window cleaners, (Justin is one) that have built up a very good round using only leaflets, though it does take determination and a little more time than canvassing.
-Philip