Markybop,
If you just clean domestic properties, I would consider magnetic advertising signs that you can take on and off the van or car when you like, just have simple wording like `WINDOW CLEANER`followed by your telephone number underneath, and your name.
Its also a idea to maximumise your advertising by wearing those highly visable bibs/vest over the top of your workwear, these could be printed with your company name on it.
Its also a idea to place a leaflet or card holder on the outside driver or passenger window for people to take your litriture when passing your vehicle.