Test phosphates at least once a month.
Don't buy equipment on an impulse - especially new "stuff". Let others be the testers before you go out and spend a lot of money on something that really isn't worth it.
Fish jump - even ones that you think would never jump. Yes, rimless tanks are beautiful but, fish losses due to jumping are just heartbreaking.
Name your fish - it seems dumb but, if you have a personal bond and a name to go along with the bond - you will care for them "just that much more".
Always have water on hand - both salt and fresh - you never know when you will need to deal with something at 2 am on a Sunday morning.
When first adding corals - don't buy the expensive, got to have it, trendy designer zoa,sps, acan etc.. buy someting basic and if it thrives for 3 months; then you can progress to something else.
Avoid GSP unless you want a GSP dominated tank.
Know your supplier -do not blindly trust anyone, not even your LFS - they may not know that the rock they just got in is loaded down with red bubble algae, AEFW, red bugs etc..
Bayer is a cheap coral dip - use it religiously on everything.
A wifi camera on the tank is an inexpensive insurance policy / piece of mind.
Local clubs are a wealth of information.
Think in years not in days. Yeah, that tang is small and cute now but, think about what size it will be in a year.
Go diving / snorkeling on a true reef as it can serve as an inspiration for your tank - we all strive to have these showcases of coral and marine life but, only (maybe) .5% of the reefs in the world look like what we are striving for in our tanks.
When you are going on vacation take time to explain the tank to the person who will be watching it - I mean every aspect of the tank down to the breaker it is on - if it can go wrong, it is going to go wrong while you are away. Have a person you can have your reef sitter call in case of emergency (either service company or reefer friend - this is where participating in local clubs come in handy).
Back to the vacation thing - don't change anything in the tank the week before you leave - you may be good but Murphy is better.
There is such a thing as too much light.
Your eyes are good at telling something is wrong but it never hurts to test.
A simple dosing pump is one of the best investments you can make for a reef tank. Stability is so very important in this hobby.
What works for one reefer may not work for you.
And finally - realize this hobby is art, science, prayer, witchcraft and dumb luck all rolled into what I call... Reefing