Never piss off the people who handle your food, set your schedule or service your airplane.
You'd think that was obvious, but I'm sometimes appalled to see how pilots treat support staff. A little goes a long way. It's not the dispatcher's fault that she had to call a crew at four in the morning. Yell at her now and then wonder why you have so many five am starts on next month's schedule. If you need de-icing service again tomorrow, which crew will respond faster and do a better job, the one you greeted with "it's about time," or the one you brought doughnuts for? Most people in the aviation environment are very professional and wouldn't deliberately do anything to sabotage a flight, even if you left them seething with rage, but they might not lift a finger to help you out when you need it, either.
I know of a bizjet crew who routinely added a pizza for the ramp workers onto their catering bill. That's a trivial fifteen bucks, giving them priority service every time. And you never know when the eyes, ears and goodwill of the ramp crew can save you a lot more than the cost of a pizza.