Dont do anything for free like that, you will just get used by many!
I started out 6 months ago and couldnt afford to do any 'offers'. My wife and i put the hours in knocking, leafleting and chasing up messages & going to quote people.
Ive had loads of refferals from my customers and they still keep coming (stopped knocking & leafleting after 4 month graft)
Best thing i would say is offer your existing customers the 'chance to get there windows for free'
If they can find you say 3 customers, then after there first 3 cleans, you clean theres for free or clean the insides. Or something along them lines.
Just my 2p though