This is the simplest method and the most effective, get a friend or trainer and stand face to face as if you were boxing, now throw a jab, every time you drop your hand your friend/ trainer slaps you and the more you do it the harder he slaps (within reason) . Also when on pads ask the friend to hit you if you drop your hand.
This works better on the pads as it is more realistic.

Honestly it is the best thing to do all the time, anytime a boxer drops his hands on pads i give him a slap (even a light slap gives him a wake up call). Otherwise your wasting your time when you could be teaching them something useful your letting them endanger them self's when they fight

I would rather be slappeed a thousand times in the gym than, punched in a fight because of a sloppy guard

Go light when slapping some one, i am wearing mitts when i do it.

HeHe not many people drop their hands when i am on the pads