Done right, you can often get your new customer to qualify your next prospect ... and if you are confident enough, you can get your new customer to get you an appointment! 
At conclusion of sale in a residential environment:
'Thanks for the order (or whatever is appropriate).
'I'm going to be calling on a few of your neighbors. Would you mind if I mentioned that you bought your (whatever) from me? Your customer doesn't mind.
'Thank you. Specifically, I plan to talk to the McClintocks on your left, and Wortmans and Gradys across the street. Of those three, who do you think would be most interested in (your product or service)?
Assuming your customer names one of the neighbors, you can just say thanks, or, if you have very good rapport, you could continue:
'Do you know Mr. Wortman well enough to call him on the phone?'
If yes, 'Would you do me a favor and call him and tell him that your are pleased with the service I have given you and as a favor to you, would he see me for a few minutes?'
