I normally tell them "thanks for volunteering" (even when I know they haven't). I ask them for a big strong flag to show ball in and out of play, throw-ins, corners and goal kicks. I normally tell them that I will go with them whichever way they decide, but if they see me indicating a different direction and see that I am not changing, they are to reflect my call as I may have seen a deflection that they may have missed. Offsides, as above, really. Indicate to me if you see a player in an offside position and they receive the ball. I may not go with you if I feel that I am in a better position to make a more informed decision. I also tell this to the captains and tell them to tell their players to play the whistle, not the flag.