Ben,
canvassers who are selling services to window cleaners normally ask for 2 cleans , sometimes less but 2 is the most.
dont forget customers cancel or become one offs so take that into consideration if you are striking a deal with one,
and be carefull of unrealistic guarantees just think if you would be able to offer what you are asking for and then you have your answer.
try and take a small amount first wait till the 2nd clean then see how you feel if it worked well for both of you then maybe re use.,
dont get too excited on the first load customers will be smiling and over the moon you just removed maybe 2 years of dirt for a tenner so just wait and see what you got left on the 2nd clean, dont presure the customer into monthly or question what he wants you never really know till you have there money in your hand,you might need to call some on the 2nd clean to find out try not to get stressed out too much it is what it is a one off others will stick , and you will find out where you are.
good luck m8t
hope this helps.
oh , you should canvass yourself a bit too , gives you an idea of your area,going rates , etc etc, im sure you may already have but there you go!
and also you cant beat the customers you cancel yourself because you will see price and meet, the other side of the coin is you will pick up less cos you will be too carefull!