It's pretty much impossible to say for sure, but I think the best strategy is to dress smartly (my favoured uniform is blazer, khakis, dressy shirt), be pleasant (especially when other pax are being unpleasant) and show empathy to the GA/TA of things are rough. I try to be like that regardless, but I'm quite positive it's gotten be upgraded.

In fact, on one occasion in SCL I'm quite positive that it was my sportsjacket that brought a T/A back to me in the lineup to check my ticket and offer me an upgrade if I took another flight two hours later. Made no difference to me as I had a 7 hour layover in Sao Paulo.