I have flown business class far more often than is right for a person under 30. I have never paid for business class and only a few of those times did I use my miles. I thought I'd share what I've learned.
When I check in I always ask if a flight is over sold. If it is and I can afford to miss the flight, I ask if they are looking for volunteers. An airline would much rather upgrade people than have to give away free flight vouchers and if you're on the volunteer list, they may upgrade you instead as a "thank you" for volunteering. If at all possible, earn status with an airlines frequent flier program. For several years I had "Premier" status with United's Milage Plus program. This meant on two occassions when I flight was over sold, they upgraded me without my asking on over sold flights.
If you can't afford to miss the flight, pay attention to what's happening. This happened to me most recently in Helsinki. I knew economy was over sold, I was told as much when I tried to get out of seat 47J - middle in the back. (I tried the getting on the list trick, but they'd already requested volunteers.) So I sat near the check-in desk and watched. Sure enough, one of the women from behind the desk approached a couple sitting near me. She asked if they were sitting together and then said, "Let me see if I can get you some better seats." She returned with two business class tickets. I then approached the desk and said, "If you're still looking for seats, I'd be happy to give up mine." She brushed me off. So I kept watching and then one of the men (very handsome) came from behind the desk and again approached a couple who also ended up in business class. When he came out again, I approached him and said "Are you only looking for couples? Because I'd really like to move up. I promise to fly Finnair from here on out." And sure enough he took my ticket and after several tense minutes he came back to me and my 47J had turned into 2C. Here I am enjoying my preflight champagne:
I'll tell you, there is nothing better than being able to lay down and sleep on a long flight.