The few times we haven't tipped we have spoken to the management.
At restaurants, we always tip at least 20%; during the holidays, we give more because it is the season and we know servers can always use the money. We figure if we can afford to eat out, then we can also afford to tip. When the pizza guy delivers, we always try to give him a good tip, too, because we appreciate not having to go pick up the pizza.
Now if the service is really bad, we'll still leave something, but it won't be 20%. We rarely have really bad service, to the point where Rick won't leave anything and we try to consider that s/he might be having a bad day or something else is going on. If the service is simply horrid, no tip....that may have happened to us only once or twice, where it was that bad that Rick didn't tip something.