Aim for houses that have a bit of a complication to them. When i first meet a window cleaner i can more or less guarantee he wants me to knock housing estates that have been built in the last 20 or so years, you know the easy to do ones with flat upvc windows. Everybody wants them, hence those estates get saturated with window cleaners. I do knock them and get work, but you have to be confident in what you are saying to get them, whereas the complicated houses are easier to pick up.
If you go for estates with bays, especially five sided bays, some maybe leaded windows, they may look a bit complicated, those type of estates don't get canvassed as much. Most window cleaners try to get houses that have flat windows with no complication, if you are a little different and keep plodding through the abundance of "no's" you'll pick up work.