Ladies and gentlemen of the lands of Tyria, wear pants.
If I could offer you only one tip for adventuring, pants would be it. The practical and aesthetic benefits of pants are obvious, whereas the rest of my advice is rarely practical and of subjective aesthetics. I will dispense this advice now.
Enjoy the power and beauty of Pre-Searing; oh nevermind; you will not understand the power and beauty of Pre until the Searing. But trust me, when you're tromping around the Realm of Torment, you'll look back at the charr of the Northlands and clear a character slot for your new perma-Pre.
Your build is not as cookie-cutter as you imagine.
Don't worry about nerfs, or worry, but know that worrying is as effective as taking on Shiro with a 55hp monk. The real trouble with maintaining your build is noticing every new skill hidden on some obscure trainer in the middle of some Dwayna-forsaken rock that could make you 10 times more powerful.
Kill one thing every day that takes you to 60dp.
Don't be reckless with the aggro. Don't put up with PUGs who are.
Don't waste your time on farming. Sometimes you'll get drops, sometimes you won't. There are plenty of greens, and most of them can be duplicated with collector items and upgrades anyway.
Friend the polite, useful members of PUGs; put the jerks, idiots, and spammers on Ignore. If you find the former, pass their names my way.
Keep your first set of armor; throw away your starter weapon.
Don't feel guilty if you have more skill templates than Kuunavang has scales. The most interesting Canthans I know couldn't decide on one build for each character by the time Nightfall came out. Some of the most interesting Tyrians I know still can't.
Get plenty of keys.
Be wise with your ID kits. You'll miss them when they're gone, your inventory's full of gold un-ID'ed, and a green drops.
Maybe you'll take Jin, maybe you'll take Sousuke. Maybe you'll take Margrid, maybe you'll take Master of Whispers. But if you choose Goren over Norgu, I'm sorry, there is simply no excuse.
Enjoy the quick-access weapon sets. Use it to switch up damage types. Don't be afraid to put a joke weapon in one slot.
/dance *, even if you have no one to synch with but your own heroes.
Browse the wiki, even if you don't register.
Do not read the Recent Changes: they will only make your head spin.
Get to know your guildmates: you'll never know when the guild will disappear forever. Be nice to your rl friends who play: even if they're not the best at the game, at least they'll go on suicide missions with you. Plus you know where they live so you can go hit them if they over-aggro. Don't you wish you could do that in PUGs?
Understand that friends come and go. And come and go and come and go and come and go, in many cases.
Be a warrior once, but remember to let the spellcasters recover their energy. Be an assassin once, but remember that you are not a warrior.
Be a Cartographer.
Accept certain inalienable truths: system requirements will go up, developers will nerf, you too will be an oldbie, and when you are, you'll fantasize that when Prophecies came out you could run the game on a Commodore 64, all the skills were perfectly balanced before devs started nerfing, and Gaile Grey never made a mistake.
Cut Regina some slack. She has a different style from Gaile, and company dynamics and such have changed. And no, you shouldn't go running to Gaile for community stuff even though she's still around. That's not her job.
Don't expect runs. Maybe you'll stumble across someone running from Old Ascalon to Drok's for tips, maybe you have an extremely helpful guild, but you never know when either one will get tired of your ungratefulness and leave you high and dry.
Do mess too much with the options during character creation. It's your one chance.
Be careful who you let into a PUG, but be ready to accept a different idea too. No profession is inherently useless, people can use their secondary professions, and cookie-cutter teams are used far more than they're worth.
But trust me on the pants. Even if you have to spend money.