I try to pitch my ladder 'above' the window and if I can achieve a safe angle will clean the window.
If I can't do that, I try and get it into a position where my missis foots my ladder.
If after trying both of these, and it's still not safe, I explain (and show if possible) to the customer why I won't do the window.
I've fallen, once, while doing one of these windows and it was a lesson that I really don't want repeated, as it gave me quite a 'shock' to say the least.
Luckily enough I'm a Geordie, so just bounced, but if you're a Cockney - it'd probably mean a year off work or so.