One single piece of advice I would have given to my high school self:

[randomness]

"You have to ask. If you don't ask, you won't get."

You still may not get what you want if you do ask, of course, but if you don't ask you almost certainly won't.

Comments

[whitebird.livejournal.com]

My phrasing for it is, "If you do not give people an opportunity to say no, they will never say yes."

[r-ness.livejournal.com]

Well put.