A head butt could come several different ways, so I'm wondering what sort of positioning was involved.
1) If at shirt-grabbing distance (and ripe for a good butting), the easist answer is headbutt him first (so that means back to more training!!!), or at least move into....
2) The clinch: Grabbing the back of the head is what many inexperienced head-butt-ers do. You, being a martial artist type, should keep two things in mind when in a good clinch: first, move in to keep your face jammed up in his. People don't like to have their face rubbed and this sets his motion going backwards. second, attacking with the knees from a clinch should be second nature. at the very least this keeps him busy.
3) If your head-butt-er came from a distance, as if he were playing soccer with your face, well, then, you're dealing with a spazzed-out nut, and there's really nothing you can do with those guys except for sheer reactions.
But really, hindsight is 20/20. These are merely suggestions for further training. And during bar fights chances are you are going to be hurt, but decisive action to create distance (or to put the beat-down on them) will make a big difference.