It really is an art lol.
I am not sure what you want but if it's just sex, what I'm learning is that guys (mostly ESXX) are rather simple...'act' confident and tell them what you want. Don't worry too much about rejection, etc. I'm experimenting with this assertive behavior now and it works...very well. The only reason I'm doing this is because I'm not in my usual feely state so I know I won't become attached.
um...dating...uh cant help much there be/c im still trying to figure it out. but most recently, i ran into a guy that i have a huge crush on at a party on friday and instead of being my usual self (running away and hiding), I went up to him, smiled a lot, and we ended up talking alone for awhile after the party (for people that i actually want to be in a relationship with, I dont initiate intimacy right away). So once again, le confidence. I have no confidence whatsoever lol, but I can put on an act. Try it.
lalalala ummm yea, I think that's about it. ALSO, when you go out, wear something that makes you feel beautiful

your favorite dress, makeup, etc. It really does help boost confidence.