You can always lie and say it's your birthday and beg to go backstage. It's incredibely pathetic, but there are actually people that do that and get back.
This is just my opinion, but unless you know someone at the theater or in the show and are honestly going there to visit or whatever, you don't really belong backstage. Personally, I would feel really uncomfortable if I were back there and didn't know anybody. Friends belong backstage; fans belong at the door.
"It's the little things; the details, that distinguish the Barbra Streisands from the Rosalyn Kinds."~Gilmore Girls~