Listen to the local radio station of where the concert is for contests. Often there might be one that includes backstage passes. Fanclubs also might have contests for backstage passes.
Lots of venues have special tickets that can be purchased (cost a LOT of money though). Or you will need to jump security or make friends with important people. haha
The best way to get back stage is to call locate radio stations and ask if there is anyway to get backstage. Offer to write an article of your experience.