I've never been in a bar fight, but most of my fighting stories are pretty humorous so I'll share.
The first one was at a San Antonio Spurs Basketball camp when I was 10. I was really small for my age, but there was this even smaller kid in my group who obviously had early onset small man syndrome. He was running his mouth all week, and finally on the last day of camp he set me off by saying my grandma could probably shoot better than me. I started punching him as fast and hard in the stomach as fast as I could while tears are streaming down my face. He never hit back, and the coaches broke it up and moved me to a different group to finish out the day.
The second one was in high school. I was on anti-depressants, which is pretty much the dumbest thing you can put a teenager on when their hormones raging, and they were changing the doses on me trying to find the right mix. They prescribed me a lower dosage I was supposed to take more doses of a day, but I accidentally kept taking the higher dosage while upping the doses per day. It's hard to explain it, but pretty much anything that might annoy you or upset you even slightly set me into a rage. A bunch of us were playing wall ball against the auditorium, and some kid did something to pull the pin. I pushed him and he pushed me back. Everyone gathered around for a fight, and he slapped me. His slap snapped me out of it, and some of my friends grabbed us and held us back. The other guy yells "I kicked your ass!", and someone else goes "You didn't kick anyone's ass, you b*tch slapped him."
The last fight, and only real fight, I was in was a muay thai fight at a gym I trained at. I found out that I don't enjoy getting punched in the face, and it's a good idea to spare before being in a bar fight. It's also not a good idea to fight a guy who's been training for a year when you've only been training for 3 months, but of course I didn't know any of that before the fight.
My only bar fight experience happened on graduation night in college at a bar/dance hall in College Station, TX. My friends and I were all out dancing with our girlfriends, and a fight breaks out right next to us. I spin around with the intention of protecting my girlfriend from the fray, but my eye catches one of my best friends who has forgotten his girlfriend and is just trying to get some free punches in on the random guys fighting. It still makes me laugh.
Bas Ruten has good and hilarious advice on how to with a fight. I can't post the link, but just search for "Bas Rutten Instructional Video - the funniest" on youtube and it'll be the first video, it's seriously funny stuff.