I've read here - and believe it to be the case - that the best way to get recommendations is to ask for them.
I always ensure my 'first cleans' are top notch and say at the end something along the lines of, 'If you think we're great, please recommend us'.
And on all my 'Your Window Cleaners Have Been' chits, I have 'If you like what we do, please recommend us to your friends, neighbours, and work place'.
It does work; asking for recommendations.