Next time your getting t-shirts printed, why don't you try it with a smart phone bar code. When your cleaning windows, people interested in future window cleaning of their residential property but don't have the time to speak with you or get a card might just scan your t-shirt.
They get your website up on their phone then bookmark it for later.
They can be from the other end of the street and do this, sometimes you might not even know it has happened.
Then to establish if its working for you - when ever you get a call ask them how they got your details. Keep a record and compare this against your ROI with your leafleting or other sales campaign.
Cool hey.