This can never be said enough times. Being really truly grateful for something should be the only time this is said. I believe it is totally and completely undermined in meaning, along with these other three. An overused word loses value. I want someone to really feel appreciated when I tell them "Thank You." because in reality, it did mean something very important to me.
I abuse this statement entirely too often. Often times in a situation you do one of a few things: do something you shouldn't have. accidentally hurt/cause harm to something/someone. mess up. etc.
Behaviors and actions are dismissed by saying "I'm Sorry." Sorry means you feel guilt. Hurt by your actions. Disappointed. And willing to never do what you did again. Saying "I'm sorry" for stealing something of your friends, or for breaking something, or for saying something rude DOESN'T CUT IT. Only if you really mean that you are never willing to do that sort of thing again, should you say it. Mean what you say, and say what you mean.
--I Love You.
I can't push this one enough. Don't ever tell someone you love them unless your over the age of 18. Get out of high school and realize that you aren't in love. Realize that just because someone says "I Love You" and "You mean so much to me" means they've probably said it to people before you, and will after you're gone. Get used to the fact that it most likely doesn't mean anything.
Love is something you grow into, lust and infatuation is something entirely different, and is what the large majority of high school relationships is. The people you love are your best friends for life who you could never live without and your family. Of course this entire situation changes when you get out of high school and mature, but geared towards the high school going crowd, I suggest you don't use this phrase.
Don't ever use this unless you will 100% not back down from what you say. There's a lot of ways you can get across the message that you want to without promising.
And that is all I have to say.
Choose your words wisely, they make all the difference.