Ok my two cents worth of sharings... Here goes:
1) Stock slowly! If there is one most important rule in this hobby it has to be this one! Adding livestock slowly is especially difficult to achieve for a new reefer because he will see many things that he likes and will want to buy all. Senior reefers we grow bored of many things and it becomes less of a problem to us.
2) Avoid certain difficult species especially those that can potentially wipe out the whole tank as they are just not worth taking the risk! Some tangs and butterflies are ich and velvet magnet and i try to avoid them as much as possible.
3) There is no need to quarantine every single fishes. Some fishes are better to add to the DT as fast as possible than to stress them putting them in QT. I always lose a fish when i have to move it to QT. Maybe i am bad at maintaining a proper QT, but seriously how many reefers here do?
4) Fishes die of too many reasons and most of it unknown. I have accepted it and losing a fish is painful to me now not because it is expensive, but because it is irreplaceable. If you cannot accept it like me, then there is no need to go into rare and expensive fishes. You can find equal joy in keeping a nice reef tank with modest corals and fishes.
Happy reefing everyone!