I've found that the best way is the honest approach :
. I'd just simply say that overheads are rising, w/c's insurance has gone up, van insurance has rocketed, supplies cost more, the cost of living is going up all the time so 'I'm sorry but I'm going to have put the cost of your windows up'.
If they look unhappy then offer to find another window cleaner.
Most people understand rising costs.
Also, we've never sold a single job. We've always given the work away. Now, because of that we have a whole network of contacts from window cleaning, carpet cleaning, gardening, tree surgery, etc.
Sometimes we get work from other w/c's when the job is too large for them and if we get asked to do windows that are not worth it for the way we are set up, I pass the work on.
Cheers