how about 6 cleans for the price of 5 if paid in advance
THat will be fun, all your customers do it, and you work one month in 6 for free 
Just give them payment options (sae, pay on the day, bacs etc) and if they dont get payment to you and you have to collect - charge a premium, even just £1 - you will soon enjoy collecting - thats my terms for new custies!
I think your completely missing the point and this topic has gone right over your head.
as the title of the topic says discount for pre-paying customers which means an incentive scheme for customers who pay for 5 cleans in advance and get a free clean.
being as we all do cleans on 4,6 and 8 week turnaround it would be impossible to clean for free every 6 weeks regardless if your all your customers took up the offer or not.
its just like when I offer my customers a free clean if the recommend a friend which quite a few of them have done. doesn't mean I do a whole months free cleans, it means I give any of my customers a free clean who recommends me meaning every customer doesn't do it at the same time same day same week same month unless Ive just entered the twilight zone
if you pushed it on all your custys, a few will take it up and your still gonna up with half a day or more working for free, so no ihavent missed the point, i arrange a price, they agree to pay it, simples, with all the money i lay out already im not gonna go round giving them free cleans for doing something as simple as paying, whats next a discount for them being in -
And no im not being stingy or anything, i just dont see the need to give out free cleans, your still gonna end up collecting the others, might as well keep your money and just knock the extra few doors
As the dragons would say "im out"