No backpacks are allowed in our local concert venues since 9/11. If a female brings a purse she must stand in a seperate line to have it searched. So I also bring as little as possible.
One thing I've learned to bring though is bottle caps. When bottled water is sold at a large concert (at least locally) they remove the caps so that if you throw it at someone (or at the stage) most of the water comes out and it won't hurt. I've learned to stash bottle caps in various parts of my clothes because once the concert starts, someone is bound to knock over your open bottle spilling all the water

Kathleen