It's called 'Market Referral' and is arguably one of the best ways to generate new business, companies like the utility warehouse use it as their core business model.
The ethos is brilliant and simple, someone recommends a good service to a friend (word of mouth) and gets a reward and their friend gets what they wanted and so on. In business terms its the same shape as a pyramid scheme the major difference being there is a product and everyone wins for a change
Simples
It works very well indeed, just don't get carried away with the sales pitch about earning 100k a year referring people
Why not contact your local trades and offer to drop their cards with your customers in case they ever need them, and ask them to take a few hundred of your cards in return
Only recommend trades you trust and know to be good, that way your safe in recommending them and hopefully their satisfied customers will place equal trust in their recommendation for you to clean their windows.
Costs you bugger all to try out to