all my customers are aware that they are very welcome to reccomend me to local friends / neighbours / family / work mates and work place , and they regularly do . i find a genuine thank - you is all thats required .
you could put on your ' windows have been cleaned ' slips , something like : reccomendations always welcome .
i wouldnt do free cleans , but if it works for you , go for it ( but they would probably reccomend you if your any good , any way ! ) .