I would agree with all the above.
I started 6 years ago, it has gone well for me, but that's because I have worked very very hard at it. I knew several window cleaners so went in with my eyes open. So here are some things I did or would do if I had my time over...
1. Try and build up some work b4 you go full time window cleaning (maybe weekends?) that way you will see if you like it, and it will help soften the loss of earnings at the start.
2. Have some savings, this is gonna take some time to build up, I did a lot of canvassing and built up some work, but it's all paying out when you are setting up, and had to use thousands of my savings, and I mean thousands.
3. Get an accountant.
4. Realise that all the things your boss does for you, you will have to do. Sick pay, holiday pay etc etc etc. Right down to your pens and paper! and remember: No worky no money
5. If you do decide to do this, it will be diffcult/impossible to go back to your old career and be taken seriously.
6. Take note that all figures quoted on this site should be subject to a BS filter. If someone says they made £40 an hour from a job that may well be true, but it is usually one job once a month, and that figure takes no account of the costs of running a business. If you at present earn £15 an hour you cost your boss twice as much, trust me, you do!
7. It gets very very cold, so cold you hands hurt like hell, but after a couple of years you don't notice as much.
7. And as someone said earlier you will only get out what put in.
8. I love it and wouldn't go back to any other job.
Sorry it's a long post, I'll shut up now