What I wouldn't do is just go round and try to take other people's business - that's really off in my opinion. Your best bet is to try to buy people's rounds. People sell when they're quitting, retiring, have too much work or whatever, and generally you'll get work in one area as a result, rather than the odd bit here and there (petrol soon adds up when driving between jobs).
My dad's sold plenty before in the Star and free ads type papers. There's generally always work for sale, but check it out carefully before you buy. You wouldn't want to end up with a round where the guy selling is getting rid of annoying customers that are underpriced for too much work. It does happen.
Also, make sure you know what you're doing. It isn't as easy as many people think, and it's a bloody hard job. In winter, it can be an absolute nightmare, but as you'll be self employed, its either work in the cold or dont get paid. A lot of new window cleaners under estimate how hard a job it is.