How we change what others think, feel, believe and do
Customers can be rude and unkind, which can make the server feel like they are losing their dignity. They may also be mean, leaving no tip ('stiffing') or saying something derisory that makes the server feel under-valued.
If you cannot say anything, you can always manage your expression. A blank look can say 'I know what you are doing'. You can stare in incredulity at a small tip. Be careful about how angry you look -- a complaint could cause you problems.
If the customers are unpleasant or look like they are not going to give a tip, then you can just downgrade the service, maybe not pouring water or wine or advising on the menu. Spend your time with other customers, where good tips are more likely.
Killing them with kindness
One way to handle unpleasant customer is to be overly nice to them. Counteract their sullenness with cheerfulness. Ignore rudeness and do not rise to the bait. See if you can make them feel really guilty. Put on an act that would melt the heart of a snowman.
Ask for feedback
Politely ask if they are getting everything they need and are happy with the service. If they are, then carefully point out that they are being unkind (a better word than 'rude').
Don't cross the line
There is a line which should probably not be crossed if you want to keep the customer or the job. Dumping food in their lap or pouring water over their heads is funny on television but can be much more trouble that it is worth.
It is good to have pride in your job and when others do not recognize the good job you do, it can be hurtful. It also hurts when people treat you as less than human, like a robot or a slave.
Sometimes there is a mental trade by servers, where they sacrifice the tip in exchange for emotional benefits of restored dignity.
Serving food can seem close to slavery and indignities can feed low self-esteem such that a small push may send the server over the edge into anger and vengeful action. They know this can lead to loss, so they hold in the anger, and so may seek ways to leak it out carefully, though a bad customer can become a victim of pent-up frustration.
The best approach is often humanization, where both server and customer see each other as people, and not a machines or items to be processed. If the server can facilitate this process, then their chance of a better tip can only go up.
Paules, G.F. (1991), Dishing it Out: Power and Resistance Among Waitresses in a New Jersey Restaurant, Temple University Press, Philadelphia, PA.