I promised myself that I wouldn't post again on this thread but I can't resist :

Firstly working for nothing is not a concept I understand.
And as for the before and after photos: OK, you take the first photo of a dirty window. You clean it and then take the second photo showing a clean window.
Fine! But what's your point? Surely your job is to make a dirty window clean? It's a bit like Asda taking photos of customers going into the store empty handed and then taking photos of them coming out with carrier bags...."Look! we've sold them food". Or your dry-cleaners taking a photo of a pair of trousers with a wine stain and then a second one without the wine stain..."Look! we got the stain out"
But more importantly; you do it for nowt...and take ages to get the glass up to pristine condition for your photo which you proudly show to all your prospective new customers...
If I were that customer I'd be expecting that same pristine standard (per the photo) and if you didn't produce that result I'd accuse you of misrepresentation and refuse to pay (the full amount).
Now be honest with yourself: Yes, as a one off, take-as-long-as-you-like-clean you'll get a great result...but are you going to do that day in day out...no you are not!...unless of course you're going to go top-end and spend a day on a conservatory and charge £200...which I suspect you're not.
Trust me and others on here. Do a half reasonable job, charge half reasonable prices and the recommendations will come naturally.
Oh! and few people look for or indeed expect "perfection".