Like many have said, word of mouth advertising is one of the best. Offer to do the customers windows for free for each new customer the recommend. If they recommend 10 customers that's nearly £70 they'll have saved (and even more once you've put your prices up!). You may well get the odd new customer that drops off after a month or 2 but most will stay and become long term customers if you do a consistently good and relaible job.
Regards
Mike
This is a good idea, but you need to qualify it. My experience of offering the above is that a few of my customers went for it and I had a scattering of houses all over the place; properties I eventually dropped or passed on; and the customers who recommended me ended up getting lots of free cleans; one for around six months!
Qualify it with a statement such as, 'and if I take them on', or 'anyone living in your own street', or within fifty meters of your house'.