1. Always be Humble
My two favorite areas of study are Astronomy and History and if there's one thing those two things have taught me is that we are all infinitely insignificant in the grand scheme of things.

See that little speck that's circled? That tiny little speck surrounded by thousands exactly like it? That's you. That's the location of everyone you know, have known and ever will know. Every problem you've had, every accomplishment you've made are right there on that tiny little speck floating in an infinite void. Tell me that your upcoming math test matters after seeing that.
Even when speaking in strictly human terms let me ask you this; how many people have gone unrecorded by history? millions? try billions. While it's true that recorded history is getting better how will you make your mark? Invent something cool? Sure, that will be a good legacy after you die but what happens when it becomes obsolete? What happens when someone invents something better? Sure, you'll be remembered for a time but eventually you'll fade into obscurity, once a memory and now nothing more then a part of the soil, decomposing until all you are is dust.
When I remember these things it is immensely humbling and it always makes me realize that comparing myself to other peoples success is pointless because nothing any of us do will ever matter. It's actually kind of freeing as well.
Which leads me to #2.....
2. Live life to the fullest and enjoy every moment you can
So, you're destined to die, the unfortunate truth I have to accept as an atheist is that there is no life after death, a literal lack of existence. That thought used to scare the crap out of me and I started to think "Well, what's the point of all this if I'll never be remembered and I'm insignificant as a fly?"
The point? There is no point to life, you make up your own rules (hence the list) and live by them. Enjoy the little things, do the things you enjoy, never put off till tomorrow and never let what other people think get in the way of what you love. You have a finite time on this earth so you might as well enjoy it.
3. Nothing is black and white
This is something I find is difficult for some people. Just look at an average child's upbringing, they're told stories or watch shows or movies in which there is a definitive "hero" character and an "evil" character where the hero is often portrayed as a paragon of virtue and the evil character as being..... well, just plain evil. Almost nothing in reality works this way and branding people who oppose something you believe to be the bad guys just makes you narrow-sighted and unwilling to see the whole picture.
I don't see things as good guy vs. bad guy anymore, conflicts are differences of opinions and while I may side with a specific side due to their opinions coinciding with mine I always try to make an effort to empathize with both sides and understand why both of them believe in what they do.
4. There is always another side to a story
Building on the previous rule, it's never good to assume why people do things. Only listening to one side can cause you to take issues out of context and create opinions based on false or inaccurate information. Always make sure you hear both sides of a story before judging someone.
5. Create your own definition of happiness
I used to always worry about how much money I would make in the future or whether or not I'd be able to purchase enough stuff to make a happy living. I never really figured this one out from experience but rather I picked it up from what other people have taught me. I realize my favorite things in life don't require a ton of money apart from travel which is something you can spread out over a period of time. I now worry less about getting a job that I can make piles of money on and more thinking of a job that I'll truly enjoy until I retire.
Well gee, I kind of just drew up a lot of these on the fly but they pretty much speak to me in the truest way possible. Good topic, anoni, good topic.