The biggest mistake among inexperienced travelers is that they complain about what's wrong without explaining what would make it right. If a trip needs to be rebooked, look up new flights on your phone while you wait in line and suggest the itinerary to the agent. If you're unsatisfied with a room, tell the hotel manager you want free breakfast or a discounted upgrade.
Be sure to keep such requests reasonable. For example, an airline generally prefers to rebook you on its own flights before paying to put you on another carrier. Presenting solutions to company representatives makes it easier for them to keep you happy while reducing their own stress – and that's a big win for everyone involved.