Some run a loyalty card, one of payment to top up. Incentives like spend X amount a month and receive a free Sunday dinner. Or top up 30 pounds on a Sunday during the football and receive a free pint. Pints at a lower price etc
I worked in a few bars when I was younger. A tip would of been normally get yourself a pint as well. I would keep a glass or jar at the till, the value of a pint was added. On a busy night you could earn more than you got paid for the shift.