At my old job I was a lunch regular at a Johnny Rocket's burger joint across the street.
My usual order cost about $7.50, for which they'd get a typical 15% tip from most people. (I once saw someone count out coins to the penny, $1.12).
I liked the servers there, and the two guys who usually served me at the counter were both really cute, so I started leaving the change from my $10, a 33% tip.
It got to the point where these guys would practically fight over which of them got to serve me. They always made sure my burger was exactly the way I liked it, that the fries were always freshly made, piping hot and crispy, my drink was refilled without having to ask, and it was as close to five-star service as you'd ever get in a chain burger joint.
All for an extra dollar and a quarter. Funny how little it costs to be nice.