Hi
Just ask each of your customers when you see them if they have any friends who are looking for a good window cleaning service. If they say they will but don't, ask them politey again the next time. It's in their interests to help you expand your round, as if you can't make a living from window cleaning then they will have to look for a new window cleaner. You don't have to offer incentives, but a quality box of biscuits/chocolates is always a good way to say thanks.
I recon that 80% of the business we have acquired in the last 5 years WFP has been with referrals. Some customers are good at it, some won't give referrals.
But the thing is that your standards of work have to be good to earn recommendations. The only down side to this is that you must expect your customer base to be very fragmented as friends are not usually next to each other.
If you get a new customer in an area of town you haven't done before, canvass a few doors either side. Remember, it's your customer's windows but it's your advertising board.
It takes time.
I am always amased at the number of people who know who I am but who I have never seen before in my life. We have 2 vans, father and son business and neither of the vans are sign written. People talk.
Spruce.