The easiest thing to do is like the other guy said there is to say your window cleaner sucks. If the person you are talking to has half a brain your done.
Find reasons why he needs your services without tearing the other window cleaners down.
Notice how others conduct there business.
Do they smoke on the job.
How are they dressed.
Do they interact or get in peoples way.
Do they come at proper times of the day.

Set yourself apart in these ways and use this line of thinking. You will create a dignified professional relationship instead of a kindergarden mentality I think the other guy sucks.