Give your best customers something of real value to offer to their friends (a secret menu, a significant gift certificate). Once you do that, not giving that gift to a friend feels selfish. Paying me to refer you rarely works, because you're not just asking for a minute of my time, you're asking me to put my credibility on the line.
If you really want to generate those referrals, don't ask for a referral, ask if everything was great. Offer to help.