For those who suffer a bit when their muscles cool down after work, I have picked up a couple of helpful tips. It may be that many people are aware of this stuff already but no flaming, eh guys?
Within 10 minutes of stopping work for the day, go around your body and tense up each muscle group in turn. Tense then for about 20 - 30 seconds each. Don't strain anything or overdo it just a comfortable tightness is fine. I find that this helps to vastly reduce that aching in the muscles that often comes several hours later. This may not be so applicable to younger window cleaners but as we get a bit older, the muscles tend to start losing their elasticity.
Knee problems? Sometimes I get a knee making a protest :-)
A method that often works quite well for me is as follows:
Stand facing a wall a few inches from it. Raise the toe end of the foot so that the heel is on the ground and the toes are resting against the wall. Gently move the hips towards the wall. If it's being done right, the back of the thigh muscle should start to feel tighter. Hold for 20 - 30 seconds and repeat several times with each foot. DON'T OVERDO IT. This often helps to alleviate pain in the knee. Obviously this is just a stopgap. If things persist too much it's better to seek medical advice.
I suppose I will get the p ripped out of me for these tips :-) but I have genuinely found them helpful at times.