Who you know. Back (waaayyy back) when I was a producer I used to get them just by asking other producers. Work your contacts. Remember the old 6-degrees-of-separation thing, and start asking around. SOMEBDOY you know is friends with, or related to, or lost a lawsuit to, someone who knows the right people. Find out who is producing the concert and start asking around for a connection. Be nice, be polite and nd be shameless.
Also, the classic method is to get to know the security staff at the venue. If you're a girl - well, if you're not, use the old standby who-knows-who.
Press passes are also a good bet. Immediately volunteer for the nearest paper and write all their concert reviews, or do photo or internet stuff. Heck, you even get free tickets if you get into the routine.
And, do you know anyone who works in marketing-publicity? That's where most of the perks go. Work those connections, baby. Also, these days, lawyers for the rich & famous.
Also. Restaurants make deliveries, florists, massage therapists, preachers, ambulances and so on. A lot of places have "approved" lists & lists of people they use regularly. Be creative. Join a union (or do you know anyone in a union? I used to "apprentice" at Madison Square Garden.)
If all else fails, become a producer (easier than it sounds - trust me), contact the band's people and tell them you're interested in (make up something sexy) "exclusive rights for using the digital code to their music and printing the code on T shirts" - you'd like to meet them backstage and discuss it. Works every time.
Good Luck, hope this helps.