At the club I shoot at there is an ON/OFF and release switch just inside the right side of the trap, these are reachable before you get in front of the house. the right side as viewed from the line.
So how about, in two inch bold lettering!
Turn off and then release the arm before getting in or out of the house and when done for the day!
3 years ago we had a nice lady sporting one heck of goose egg on her forehead, she opened the house, took a couple steps back, then the kid getting in the house to turn on the hydrulic pump brushed the release switch releasing the unreleased arm, the bird that was thrown took the gal about an inch above her safety glasses!
so also add
never ever stand in the line of fire! those little clay discs do more then sting!
There are two obvious things, that have been mentioned above (release the arm/use a flag or other device sticking above the house).
A third safety procedure that is a little more difficult to work out happens frequently at some shoots. A loader raises a flag, gets into the house and then in a minute or so, lowers the flag. I am not sure if the loader is going to remain in the house while we shoot a round or if he/she just accidentally knocked the flag down.
We always ask the scorer if the loader is going to stay in the house and we hope the scorer knows what the other person has planned.