Construct a fake backstage pass. You can show up to the concert early and get a look at what the real backstage passes look like. See if you can get a photo of it and then use your Photoshop skills to make your own pass to sneak backstage.
Make yourself part of an entourage. If any famous people are heading backstage, then they likely have a group of personnel with them. Walk backstage at the same time as the group and try not to get caught.
Dress as a crew member. People who work a concert may wear a certain uniform or dress in all black. Copy their look as a way to get backstage.
Exude a sense of belonging. Most bouncers catch people trying to sneak backstage because they make it obvious that they shouldn't be there. Avoid this by looking confident and relaxed.