Northern Virginia, D.C. area. It's a seasonal job but you will do just fine if you do everything yourself.
I started doing individual private homes, then moved on to working with realtors and community organizations. Best bang for the buck is just going to few apartment buildings and dropping 200 pamphlets saying "Window cleaning on Saturday: Call 1-800-xxx-xxxx to schedule. $100 Cash/Check". If you're doing move outs, take cash up front. Don't ever ever have credit card on file; they will give you the card, then go shopping online. Don't be lazy and hire someone off the street: some homeowners are social parasites praying on these types of businesses, if they sense you're an amateur they will claim theft or broken property to get a free service, or worse, hustle you for money! The worst ones are middle-aged single men. If you see a lawyer don't bother serving them; scum of the earth, $0.10 is the price of their letterhead, and it's how much it costs them to ruin you. If you suspect someone is fussy, walk away, not worth the trouble. Be firm and do good job.
Don't use a squeegee by itself, squeegee then wipe. Thin roll paper is best, the kind that don't feel spongy like a toilet paper (but more like newspaper.) If you use cloth rags get the thinest hardest ones you can get; the padded ones with sponge or cotton inside wear and tear after wash and the cotton comes out, sticking to staticy glass.
No poles or extensions. Period. If you use a ladder watch where you lean it against, and watch where it stands (I had it sink into soft ground slowly throwing me 12 feet down.)
Try not to break anything. Repair will ruin you.
Lock all the doors and windows when you're done and instruct the homeowner to do a check around. Tell them it's their responsibility.
Ask me if you need further help.
Good luck!
[Edit:
I must add this. No intellectual conversations with clients. People don't like "well read, well traveled" types, and if you appear upwardly mobile at all, people will feel uneasy; they get self-conscious and they don't want to be seen by someone who might be critical of them intellectually or professionally. When you see a shelf of Dan Browns, the ever pretentious Hegel for Dummies, or those "Prague for Bachelors" books, keep a straight face.]