Craig,
do you have a free local newspaper? Put an ad in it and you should get responces from areas where there is no window cleaner. Then go and leaflet that area after giving the person who phoned a quote and an incentive: eg) "if you can find me 5 customers, I will give you your first clean free..10 customers and you get 2 free cleans" etc...
Do you have a local radio station who has a DJ who caters for the older generation who like listening to the old dance hall classics...?? If you do, find their site on the net, these usually have email contact details...email the DJ saying you are starting out in a particular area and are looking for customers...it worked for me!!
Definately knock on doors...it is a thousand times more productive than just putting leaflets through the door...leaflets have their place, but ensure they look professional and reflect on your image.
Ask other window cleaners if they have any work they want to pass on.
Find a USP...unique selling point...that sets you apart from the rest and sell it to your potential customers: eg) cleaning frames, inside cleans, public liability insurance, landline contact number, WFP/WAHD, etc etc.
Look the part...if you are selling professionalism make sure you look professional...park the clapped out banger round the corner and walk your potential area.
Above all, keep logging in here for motivation.
Best wishes
Steve