The most important thing that I have learned is to BE PATIENT.
If you hurry to get your fish, then chances are you will have water quality problems leading to dead fish. Or, if you hurry to get a fish before researching that fish (the impulse buy, "I love that fish and want it now!") then you may be buying something that will eat your corals or be attacked by other inhabitants.
You have to be patient with everything you do in the fish tank, and I suppose this ties in with DO YOUR RESEARCH FIRST.