As someone who often voices my opinion without concern about popularity,
I had the experience growing up that you describe. Here is what I think that parents can do to instill a sense of personal power in their children:
1) Never take advice on face value. Always do the research to know the facts.
2) Don't let someone else blow out your candles. Keep your own dreams alive and reach for mountains no matter what anyone else says.
3) Be true to yourself. Know who you are. Don't ever let anyone tell you differently that you aren't good enough.
4) When you are up against people who say you can't, try harder, for in my experience I'm always told I can't do something when in reality, I can.
5) Know when to stop chasing a rainbow, but always have a back-up plan, and a plan for your back-up!
6) Be the change you want to see. Act it, do it, and live it.
7) Stop being your own worst enemy. If someone is not good for you, don't chase them. Let them go on and live their own life. There's a lot more to life than the one who got away or wouldn't be your friend. (Avoid negativity and people who constantly bring you down.)
8) Don't prioritize the needs of others if it is detrimental to that of your own. Meaning, it's ok to give gifts if you have it to give. But don't feel pressured by anyone because they feel you have to do something for someone else.
9) You have a voice. Use it! You have to become your own advocate. Treat yourself like you would treat someone else. Kindly. Don't beat yourself down. Make sure you are always looking out for your best interests, because others won't necessarily.
10) Don't give someone a fish, but teach them how to get their own! (From a Chinese fortunate cookie.)