I haven't been in a fight since grade school.
Choose your company well. You go out with rowdy, testosterone-ridden friends, people with bad physical attitudes, things have a higher likelihood of happening.
Walk away from trouble. A wounded pride is easier to salve than a wounded body.
Be confident. I know others have said it, but it's true. It's not that you'll scare anyone off. You'll just be a less likely target for an intimidator.
If physical action starts:
Leave, if possible. See "walk away" up there.
If you're in public, make a scene. Draw some attention.
Learn some simple tricks of the trade. Pressing to the right place in an armpit gets their attention. The ol' kick or knee to the jewels. Pressure point behind the ear. You can leverage them right down to the ground with one hand in their hair or on their head and one on their chin. A finger under the nose, with upward pressure, takes away their concentration. The knee, if struck with enough lateral force, will distract them.
None of those are really incapacitating (except maybe the jewels)(and the knee, with enough force). What they do is give you time. My kids knew all this stuff before they were teens.
And if you're fighting for your well-being or your life, there's no holding back. My self-defense instructor in college, the most skilled fighter I ever met, including the later Marine years, said, "If someone is trying to take away your rights, they have forfeited their own." Made great sense.
I like to face the door, too, and evaluate the people and situation around me, but the chance of someone just coming up randomly and assaulting me at a table is about 1 in 100,000, so I don't worry about it that much. I'm not Hickock. I haven't made the enemies that he did. Mostly for me, self-defense is keeping out of potentially dangerous situations.
Mostly. Then there was that time in D.C.......