It can also help if you're tall. Back when I didn't fly United all that often, I would check in and ask for an exit row seat. If one wasn't availabe I would ask for E+. I would say that my height (I'm 6'7'') made most other seats uncomfortable. The ~4 times I asked, I wasn't turned down or charged. It happened once on the phone and the rest at check-in.

Another winner is crutches. I saw a guy get upgraded to first, just because he had a broken foot and was using crutches. You could always try faking it...