EDIT: I forgot this thread is meant to be things one should never do, not things one should always do. My bad.
Tip a lifeguard after your pool party. They just spent 3 hours watching your kids for you, and after you leave they'll be cleaning, vacuuming, and pulling wet toilet paper off every surface in the bathrooms for the next 3 hours. Trust me, they deserve it a lot more than the waitress that carries your food 50 feet or the valet that drives your car 100 yards.
Other times to tip lifeguards: after they save your kid, or after they clean up your kid's "code brown."