A few of my favorite sayings, with their accompanying explanation. I can't claim credit for most of these, but I think they're all fantastic.
Looking good is half the battle.
This was the favorite saying of my OW/Nitrox instructor. He started saying it on the first day of class when we were going over gear, so I automatically associated it with gear for the rest of the course. eg, Having the latest, greatest, good-looking gear. It was only after the course I realized he was talking about looking good
in the water. Good trim and buoyancy, not swimming with your hands, a good non-silting kick, etc are the basic foundation on which everything else stands. If you don't have a solid foundation, when you try to add anything else onto it, you're just going to crumble.
Buy it nice or buy it twice.
This
does apply to gear. Shell out the money for high quality gear to begin with, even if it means waiting awhile to buy it, and you'll likely spend less money overall. Chances and you'll also probably enjoy the sport more if you use decent gear, and thus you'll keep diving instead of dropping out within a year or two.
When I first started diving I made the mistake of cheaping out on regulators. They were decent and plenty of people swear by them. But after awhile I wanted something better. I upgraded and immediately realized I should have just bought the good stuff to begin with.
As long as you've got air, everything's gravy.
(for those of you who want to be technical about it, replace "air" with "gas" :mooner

Breathing is paramount. You can figure out anything else underwater if you put your mind to it, as long as you can still breathe.
No dive today is worth all the dives tomorrow.
If you're not having fun, or perhaps the dive has gone south, call it. Remember that anyone can call any dive, for any reason, at any time, without fear of repercussion. Diving isn't worth dying for -- I'd rather sit out a dive today and make the dives tomorrow, than make my last dive today.
Be quick, but don't hurry.
When gearing up, especially in some locales, time might be of the essence. Rocking boat, hot sweltering sun, sneak dives, etc.

Learn to be efficient and proficient when gearing up -- the longer you stand outside in a drysuit on a hot day with doubles, or the longer you sway to and fro on a rocking dive boat, the worse you're going to feel.
However, don't hurry -- it will just slow you down in the long run.
And for my personal favorite:
Slow and relaxed, with good technique.
Might seem to contradict that last one, although this has a different application. Whatever you do underwater, do it slowly, be relaxed, and use good technique. It will make all the difference in the world. I've heard this saying literally thousands of times and I say it to myself quite frequently, both in and out of the water. It really does help, no matter the situation. This saying applies to
anything you do while diving. Also helps with other things -- school, tests, driving, etc.
One thing I've learned over the past year or two is that if you're young, people
will discriminate against you based on age or perceived experience level. It is unfortunate but I have personally had plenty of people tell me "What can you know? You're only 21!" or some variant thereof. Yes, it really bothers me -- what does my age have to do with a suggestion or any advice I might have? If the advice is sound and I present a logical argument, does my age suddenly make anything I say invalid?
I am blessed with my selection of dive buddies -- the two guys I usually dive with, while both are roughly twice my age, treat me as an equal. For that I am very thankful -- it really means a lot.