Carry fake police ID with you and flashing it is a good way to instantly defuse the situation, just offer to buy them a beer afterwards, as a reward for obedience otherwise it simmers, use this on groups when your going to get your head kicked in
Another good way which always seems to work, which I used for example last weekend in a bar when I was chatting up this guys wife and doing ...er.... certain things with her without realising he was around lol was simply giving the fight a context and making sure he didnt want to apply it to himself by initiating the fight. An example of this would be:
Hey fella look I only fight with gay guys who hit on me. (works great with total meatheads)
Regardless of what you try someone will always want to have a pop cant defuse them all, best way to think about fights in bars is it's like a bomb with a timer ticking down, whatever you do time is running out. And never strike first this saves you alot of time with the police

*cough*