Another option would be one shot for every hand salute rendered onscreen.
Yet another would be every time the words Sir and/or Ma’am are spoken.
One shot for every objection made by either trial or defense counsel in a courtroom.
Take a shot every time Harm (ostensibly a lawyer) gets an opportunity to take a flight in a jet. Two shots if he sees action while flying or if it's a jet he shouldn't be qualified on (like the Russian MiG 29 or Boeing 747 airliner from Oceanic Airlines...), not counting the brief period in the fifth season where he got transferred back into flying duty (in which case take a shot every time he ends up lawyering...)