If someone gives mediocre service, they get a mediocre tip.
If someone gives good service, they get a good tip.
If someone gives bad service, they get ratted out to the manager.
As an aside, you can get some really great stuff in the mail if you legitimately complain about bad service/product.
For example, one time my wife & I went into a Timothy's Coffee House because she really wanted a hot chocolate. The managersaid they were too busy to make it, and when I looked around there were literally 4 people in the coffee house.
Next day, I call up the public relations office (*hint* skip the customer service line - they're usually patronizing knobs) and told them how my experience soured me on the chain, and that I would be letting my friends, coworkers and family know exactly how much the corporation values their patronage.
Within a week I had over $100 worth of coupons in the mail...I think I drank free coffee for like, 2 weeks.