If you see the referees butt, you are in the wrong place!
Works well.
OR,
As a referee I tell the wings (in the pregame) not to go until I point at you, and I do so real early. That way there is no misunderstanding and if I send the wrong guy (happened once in three years
) we still have the bases covered.
This mechanic also assurs their are never three attempting to get under the goalpost! (I'm sure you know what I mean!
)