Be true to who you are.
While there is some truth in this, there are two glitches. This only works once you truly know yourself, meaning you've lived long enough to garner some wisdom about your place in the world. A young person doesn't know him/herself well enough to stay true. Also, what if you're a jerk? Or an idiot? Should we be prompting you to stay true to a version of yourself that needs to be drastically improved? If you're a moron who consistently makes dumb life decisions, or are a psychopath who routinely harms others, that is not something you should be staying true to. Know yourself well enough to know that you might need to change. That's more important that staying true to yourself.
Find your passion.
This would certainly be ideal, but it doesn't mean a lot in the practical sense. If my passion is eating ice cream and watching movies (which it definitely is!), then should I only pursue life opportunities that align with those passions? I should only look for jobs as movie-reviewing ice cream taste-testers? The world doesn't work like that, at least for most people. If you're lucky enough that your passions and your financial and civic responsibilities align, you have really made it in life. But for everyone else, being a hard worker with a positive outlook, no matter the circumstance, is far more useful. Do those first, and your passion will find you.
Live each day as though it's your last!
Ugh. This is the more tasteful relative of the imbecilic "YOLO!" If today were my last day, I would drop everything and drive to the beach, gorge myself on bacon cheeseburgers, get in a round of golf, and watch my favorite movies before falling asleep and presumably dropping dead. If I did that "each day," I would become a horrible and selfish person, and a fantastically unproductive member of society. Sure, we shouldn't spend every waking minute doing awful jobs or being around people we hate, and we should make more of an effort to spend time with loved ones and pursue joyful activities. But life doesn't always work out so neatly. Living each day as though it's your hypothetical last means you aren't preparing for your real future, which is probably more important than whatever you're doing right now. Live each day as if it truly matters, and always be planning ahead--that will help you tackle today more positively.
Be careful of dealing in platitudes when advising young people. Though you may have good intentions, those pieces of advice may not be as useful as you think.