First, a long list of YA authors who tweet! Follow one or all! I'm not on the list (yet) but you can follow me at @TinaSpringer.
And if you are a YA author, you might want to follow @YAauthors. He (or she?) only follows YA authors and will retweet your important info like book tours, new releases etc.
Check out KidPub. They follow 100 twittering YA authors and post their updates.
And finally, if you're going to look at any link on this post, you HAVE to check out Read Write Tweet! They have an awesome list of: