I think it's all been said there in one way or another.
It's not easy, but put the time in and the customers are there.
I've been cleaning for around 3 months, after closing my valeting/detailing (very high end cars, so BIG drop in business the past year due to economy) and getting window cleaning custom is much easier than other businesses. To get valeting custom I had to spend a fortune on advertising/web SEO, and regular repeat custom is harder, but window cleaning customers, it's just like any other selling - a numbers game - the more houses you canvass, the closer you are to the yes's, and hitting that lucky estate/road with no cleaner.
There is plenty of competition, but there's always a road that's sacked off a rubbish cleaner, or where someone has retired and there's still a need to fill. You'll get days canvassing with no or very few customers, but other days on the right road you can get one after another, and that's a great feeling when it happens. Especially knowing it's solid regular custom/income month after month, unlike the unpredictable building/farm trade.