Only make a thoroughly educated and informed complaint. Do not complain unless you've considered the other side and you're fully confident in your complaint.
Never be an asshole. You are dealing with real people with real feelings, and your anger is rarely their fault.
Make few demands and ask a lot of questions.
Every one of these items is vital to an effective complaint. You're walking in unprepared by only knowing your own side of the argument, shooting yourself in the foot by being a jerk, and backing yourself into a corner by making several demands. Let's look at why.