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Pretty much every other sales book out there mentions "Quid pro quo" (from the Latin meaning "something fo something").
I've recently put on my price quote emails a little extra line that says-
When you arrive check in on Facebook Places and Foursquare for an additional $50 off your quote!!
I've told the sales managers and service director when they are doing a write up they should include something similiar.
If you're going to give a discount why not get a little something for it?