First, a full 45 minutes before boarding (not before take-off, before boarding) announce "This is a full flight and we will not have enough space for everyone's carry-on bags."

Every ten minutes thereafter, continue to work the crowd into a lather by repeating that announcement and encouraging people to check their bags (not valet - check through to destination, with the requisite 45-minute wait at baggage claim, of course).

End result - no one checked their bags, extreme amount of people standing around the gate jockeying for position, on the plane, people in the back stowed their bags in the first overhead slot they saw, instead of near their seat. And they did not run out of overhead space after all!

I assume the GA thought he was doing some good, but he just made a bad situation out of nothing.