After many years of trial and error, I have found that the best way is to dress up as a giant burrito, and wait by the band's tourbus, until Boz Boorer spots you and eats you. Hidden in his belly, Boz will then unwittingly carry you past security into the backstage area, where you can eavesdrop on the instrumental soundcheck. After this, Boz normally has a little snooze, during which time you can make your escape and hide yourself in Moz's dressing room until he appears, where he will normally happily sign anything for you, as he thinks you are venue staff.
I have done this several times, and it works without fail. Good luck my friend!