Most archers (other than the occasional sneaky one
) will focus directly on what they're aiming at so if nothing else, you might be able to distinguish between a head shot, body shot, or leg shot. That should at least give you a clue as to whether to lift or move the leg, keep the shield in place, or raise it to guard your head.
One thing that I occasionally do as an archer is to lock in on a person's legs and then look up while keeping my arrow locked on the low target. When the person sees I'm about to take the shot and thinks it's a body or head shot, I'm able to hit him in the legs. The reverse also applies. I may lock in to the head and then look as though I'm aiming for a leg when I actually release the arrow.
As far as blocking, a big part of that is developing your melee situational awareness and reading where the archers are. While you still have to worry about the one(s) directly in front of you, there may also be archers farther out who may still be capable of hitting you. Getting a helm helps. It reduces your vulnerable areas. Keep in mind to try and stop the forward motion of any missile weapon be it arrow, javelin, or rock. If you don't, it could still continue on and hit someone behind you.
When you're walking or running across an archer's field of view, don't move at a constant pace. Try stutter-stepping, outright pausing, and changing your speed. A big part of applying lead to a moving target is accurately predicting where the target is going to be. If you aren't predictable, then the archer is going to have a harder time hitting you.