Some of these are things I pondered after my lunchtime open mat, some are things I've pondered in the past.
Don't forget gratitude when a whitebelt gets you. It's a diagnostic of a hole in your game and one you're likely to pay a lot more attention to, compared to when a higher belt gets you-- losing to an upper belt is more or less inevitable but is a blinking red light when it's to a whitebelt.
Don't forget to thank your opponent for rolling-- every minute on the mat is a minute of learning.
Don't use your greater or lesser weight, strength, flexibility or fatigue level as an excuse for poor technique.
Don't feel like you have to roll with every whitebelt who asks you. Trust your instincts. There's no shame in asking to go lightly.
Don't hesitate to tap. Avoid injury. Injury bad.
Don't get so caught up in competition-mode that you lose sight of impending doom. Example: let go of the armbar the moment you leave the ground.. it's better to abandon the sub with a 1" fall than to risk having it stripped from your grip and falling a foot or more onto your cervical spine.
If you're substantially smaller and/or weaker than your opponent, don't beat yourself up for failing to outpace them or out-technique them unless you have at least a two-year training advantage.
If you're substantially smaller and/or weaker, don't go balls to the wall with your technique and humiliatingly wax your opponent unless it's a tournament. There are always those who will get mad and use their strength against you in a moment of bad judgment. Inevitably they will feel really, really sorry and they might even send you flowers while you recover from surgery.. but you're still the one in surgery.
Don't let fear of injury from spazzism stop you from rolling with whitebelts-- you will never get to practice your offense or new stuff if you only roll with your betters.
Don't let snarky attitude from anyone at your school keep you from doing your best to help others. You may be able to give important feedback or constructive criticism that helps someone else.
Don't be that know-it-all bluebelt who really knows just a smidge more than a whitebelt. Discretion is the better part of valor, etc. etc. It's better to be silent and thought a fool than to open your mouth and confirm it.
Any others? Please comment with your dos and don'ts.