Lineman/judge pre-snap hand signals.

[wisref2]

Another tip - when the play comes to your sideline, get BEHIND the play! Otherwise you're going to end up IN the play.

Let's say it's a sweep to your side. Move into the backfield a few steps so the runner is ahead of you - then, and only then, should you start moving upfield to cover the play.

If the play ends up out of bounds, mark your spot while facing the pile, not the field. The spot isn't going to move, so keep your eyes on the players.