Read as much as you can on the Starting a Window Cleaning Business board too - lots of very helpful stuff over there for you.
http://www.cleanitup.co.uk/cgi-bin/yabb/YaBB.pl?board=Startingwindowcleaning NEVER undersell yourself - don't cut someone else's price just to get the job, or get fooled by the rascals who tell you a cheaper price than they've been charged. Time yourself on your existing jobs and see what that comes out to per hour - is that what you want to earn, or do you need to start increasing your prices? Do that on the jobs you pick up from now on.
Don't take one-offs, tell prospective customers that you provide a
service, your business is all repeat work and you'll be their window cleaner for years and years. Turn up like clockwork, leave them a card to say when you'll be back, when you do come back and give them the next card, tell them to pass that on to anyone they know with dirty windows or if they know of anyone without a window cleaner.
Customers are not the enemy, or people to be screwed. You want their repeat business. Be nice to them - anyone who pays me for keeping fit is definitely on my pals list. Your old customers are the depth to your business and the new ones are the breadth - you need both.
Ooh, I think I'm starting go off on a rant here, so I'll shut up now. Let me have your email and I'll send off some stuff for you to go through, if you like. Click on my name over on the left to get mine.