I would agree with Jeff and Philly. I used a combination of leaflet drop and calling next day when I was building up my residential work.
Be careful not to put out too many leaflets out in one go. It takes much longer to door knock.
You might picking the wrong areas. I advertised in hairdressers, newsagents, local supermarkets. When I got a bite I would ask them if there was already a someone in the area. Eventually you will hear the magic words. "I've not seen a window cleaner here for months". Then hit all the surrounding streets with your leaflets, door knocking next day.
Also, one of the best ways is to ask your new customers. After collecting the money. Ask them what they think of the job you've done. They will hopefully be delighted. In fact you've done such a nice job it's almost like they owe you something. Then ask them "if they know of anyone else who would appreciate getting this service." They will often give you several names. WHen you call on them, just say that their "friend, Mrs XYZ, sent me over. Said you might be interested in window cleaning" I've picked up LOADS this way. They are often quality customers who only hire contractors by recommendation. Worth a try.
Hope your business is booming before too long!