When I was 15-16 years old, I got moved from E- to E+ all the time for free. I was just being talkative and make friends with the GA and TA. Being nice and make sweet comments really helped.
I got upgraded from Y to F by GAs and FAs because one of the FAs knew my ex husband who worked for UA at that time. Another time, I gave all the FAs and GAs chocolate on my flight and was "asked" to move to F.
The point is, being friendly and sweet to people in the service industry will get you far. MAKE THEM FEEL SPECIAL.