On my way to becoming a white hat I umped for 5 years. I took the advise of an ole' vet ,who stressed read the guards as they will tell you where the play is going. His advise was:
(1) IF the guard pulls, focus on him as he will be the key blocker to the play where a foul is possible.
(2) IF the guard retreats, move up as a pass is coming.
(3) If the guard charges, git otta' town as the play is coming your way.
(4) Your most important duty is to stay safe. You don't need to be on top of the play, or the play may end up on top of you.
Oh, I got still a few bumps and bruises and enjoy R much more as the players seem to be running away from you than at you.