Trust everyone, but trust yourself more. I've noticed that people who are paranoid about everybody being out to screw them over tend to get screwed over the most. Sure, you might get burned once in awhile, but you'll heal up, and in the long run you'll be healthier if you maintain some faith in humanity.
Be kind to people, especially people you don't like. Nothing poisons a workplace or a home more effectively than petty grudges nourished over petty slights. Let it go! You'll feel better about life in the long run. Oh, and remember to be kind to yourself too.
Don't be afraid to to walk away from a job or a relationship where you are feeling pressured to do what you know is not right. Peer pressure shouldn't be an acceptable excuse for bad behavior after about grade four. Nor does " I was just following orders" carry much weight in that big courtroom in the sky. Trust your moral compass and conduct yourself accordingly.
Remember that everybody deserves another last chance. Vengeance, spite, malice, can briefly feel good while you're indulging them, but they'll never leave you in a happy place. Forgiveness, grace, and mercy can be harder to find, but will leave you at peace.
Be grateful for your blessings and accepting of your flaws. Nobody's perfect, including you and me. "But for the grace of God" is a well-worn cliche, but it holds a nugget of truth. We are all the product of a divine roll of the dice. Some of us get snake-eyes, some of us get sevens or elevens, but all of us can decide where we're going to take it from there.
Life is a gift; make the most of it!