Airline employee redditors: What is the best way to get a free upgrade on a flight with your airline?

I am flying to the states tomorrow with United, and was wondering if any of you had some good tips to get upgraded. I have never been upgraded before in my life despite travelling to the states twice a year for the last 15 years and travelling within Europe countless times. I always try to be as nice as possible to the people at the counter and on the plane, but it seems alot of the time the most obnoxious and late to check in people get upgraded for some reason.

Any other tips would also be greatly appreciated.