I carry a note in my suit that says "Shhh!" because people love to talk during performances. (And I keep it around so I don't have to make further noise by asking someone NOT to talk during the music.)
Last month in NYC I asked in a very courteous manner a woman at intermission at Merkin Recital Hall if she would mind not chewing her gum so loudly, it was a distraction to the recital. She was quite offended (so was her husband) and they ended up leaving...while I felt slightly bad that they were going to miss the rest of the concert, I enjoyed it that much more by not having the smacking of gum sounding right behind me and could focus on the awesome performance of Maria Bachmann.
Glad I wasn't beat up!