I used to do this all the time. In fact, I had all-access for the first warped tour (96) and met all of my favorite bands.
The best thing to do nowadays is start a review website or webzine. Write some opinions about records or bands. Then try to get in contact with the booking agent or band's manager. Tell them that you want to interview the band for your website. Be persistent. Let them know you are serious (even if you aren't). With a show as big as Warped, finding a gracious manager shouldn't be too hard. Also, be prepared to do the interview (tape recorder or whatever), even if you don't really plan on doing it.
This is how I wound up meeting NOFX, Pennywise, deftones, among many other bands.