Give up smoking by the time you hit 30, or by the time you have kids, whichever comes first (if you smoke obvs!) Said to me by a very caring older colleague and I did follow it, am v glad I did.
Don't be afraid to chuck it in the fuck it bucket. If something's not working don't just keep wasting time and energy on it because you don't want to be seen to give up. Sometimes you need to give up and start over/try something different.
And I don't know where I got this one but I try to live by it. Whatever you think is true/seems to be true, there's probably a way in which the opposite is true, or you can see it the opposite way round. It can help to see situations from all angles - even if you do have a firm view yourself, understand that others might see it totally differently. It makes you more tolerant and able to find compromise. Of course it doesn't apply to absolutely everything and doesn't mean you can't pick a side - but it can be very useful.
And lastly, plant a seed. With difficult or stubborn people, arguing may not convince them, even if you're demonstrably right. Instead just drop a little idea or suggestion, leave it a few days/weeks and they'll often decide it's a great idea (though they may believe it's their idea...) Works brilliantly.