i have heard of people who only charge the same amount to clean the outsides as the insides.(i.e. a house worth £10 woud be £20 for in and out), butyou really need to be charging more than that. Lets face it, people hate cleaning windows, so charge for the inconvenience.
Some people wear over shoes to make sure they dont dirty the carpets, but i just take my shoes off whenever entering a house, as it shows curtisy, and that you make sure you dirty things, afterall we are cleaners.
As Pat said, you really need to be handy with a squeegee, otherwise they will think that they can do just as good a job themselves!
I always put a scim below the glass to, because when that dirty water runs off the glass the last thing you want is for it to stain someones clean magnolia walls!