Also be boring and mention to everyone you know that you are now a Professional Window Cleaner and do they want or know where there are customers looking for Window Cleaners.
Put up adverts in you local shops, pub, chip shop, chinese or indian takeaways, newsagents in fact anywere that makes you visable to the public, most of these places won't charge you even, because you are a customer of theirs.
Also remember don't sell yourself short and charge too little, if you are confident that you are a good cleaner and you can offer a fantastic service for your customers then they won't mind paying a reasonable price for You.