There's a lot I can start off with right here, right now; a lot of surprising emotions and relative trespasses that won't matter to me in 3 months. But I can start off with honestly saying that if you love people that don't deserve it you are doing something un-human. I don't mean that in a pious sense, but by earnestly taking the things that selfish people do (or fail to do) and not letting them carry sway over your life you are doing both them and yourself a courtesy.
Remember that you can always tell a person's character by how they treat people that they don't need to treat well. Thanks Mission Impossible 3
Pray for the good fortune of your customers, especially the ones that don't deserve it. You'll probably never see them again anyway, so what good does it do to put more hate into the world?
Bad tips can always lead to an appreciation for better days. And ending the day on the smile is always better than going to bed with a heavy mind.