I tell every customer to ask their neighbours and I'll knock £5 off their NEXT clean. Sometimes get 3 in a row out of it!
One lady referred the little bookshop where she works (one of my favourite customers, really loves a good chinwag). Then her daughter called me and when I cleaned her windows I had no less than 4 walk ups, including a conservatory clean. Not really aiming for big earnings as I have found that they create themselves. That one £5 front only terrace round the corner from me came from a facebook recomendation originally.
From one single word of mouth internet request for a window cleaner I have gained over seventy quids worth of regular work. Cost in 'refer a friend' payments = £15. I actually get a lot of satisfaction from saying thank you and pressing a crisp fiver in someone's hand.