Press your boot modifier keys after the chime, but before the spinning gear appears.
One problem is that these boot modifiers seem not to work with Bluetooth keyboards, as many users who hold down the modifier keys immediately at startup find that the Mac boots up normally, ignoring the modifiers. The solution is simple: wait to press the keys until you hear the startup chime, but before you see the spinning gear at boot. This allows the Mac to load the appropriate bluetooth drivers so that it can see the keyboard, but is early enough in the boot process that the keys aren’t ignored.
Thanks, Bob, for the great tip!