I'm a martial arts instructor so I have to be quite careful with how I deal with this type of situation as I need to maintain a clean criminal record in order to pass my CRB check to be allowed to teach kids.
I'd never throw the first punch in situation where somebody was mouthing off, the clubs I go to also have very good security staff with lots of CCTV so fights are quickly spotted and dealt with. If you can stall them for enough time then usually the security will come and sort them out without any violence occuring.
A lot of people have said there's no point in reasoning with drunk people as they won't listen - I'd say that's partly true, but there are also the louts who are susceptible to a calm word to diffuse the situation. That is always my first port of call in any argument / start of a fight, very simply and very directly tell them that you're not here to fight and lets just get on with our night out.
However if I did think that somebody was going to really try to do me some damage then the moment they began to get physical I'd try to end the fight there and then as quickly as possible before it even started - the key thing being to only react AFTER they've shown the first sign of getting physical and to stop as soon as they aren't a direct threat (i.e. when they're out of your personal space). That way if CCTV tapes are reviewed you're quite within your rights to say that you tried to talk your way out of the situation but when they became physically aggressive towards you and you feared for your personal safety so did what you had to to prevent them from getting more violent and you didn't carry on with your attack with the intention of causing them physical harm.
All of that said, I can't remember the last time I was directly involved in a fight on a night out - I'm the one who always ends up breaking them up or diffusing them for my less controlled drunken friends!
In a mugging however I'd go in as hard and aggressive as possible and answer for my actions later. But that's another story.
