Being mostly commercial I haven't really tried anything. I have a friend that was doing a lot of carpets a few years ago (quit after his dad died) he had really good luck offering a upholstery cleaning to any customer that referred a friend who scheduled. there were conditions, basically nothing bigger than a love seat. Typically he'd come in and clean a chair and then they'd have him do the rest of the furniture so he made pretty good money with it.
Someone in my area was giving a 25% discount to both the new customer and the one that referred them, they racked up a bunch of new business, though I don't know how much of that turned into repeat customers.