Strike the ball with the hard first joint of the first finger, not with the soft fleshy part of the fist.

The back-spinner, which spins like a drop punt is the fastest and most accurate handball, particularly in windy conditions.

The floater is easier to execute and easier to catch on the run out in front.

The grounder is rarely used.

Handball needs to be practised with both left and right hands, short and long, running and stationary, in a 360 degree arc, standing and kneeling, to a running or stationary player, with and without a blocking player.

Practise not handballing to the first option but quickly assessing other options.

Handballing over the man on the mark to an attacking option is far better than the backwards handball, which usually forces a player to kick to a narrow set of options and puts him under pressure from the man on the mark. better to hold back and then handball over the man on the mark to your teammate as he runs forward. Evan a well-practised short kick over the top is better, with the players downfield practised at running forward to the goals, not coming up to fill that space, so their backmen are not able to block that space or put it under pressure. This is a still that needs to be practised. Sprint back and then move forward toward the man on the mark,  thereby committing the man on the mark to stay on the mark and not chase your teammate who is calling for the ball. This also gives you the momentum to get the ball to its target - it is risky to try to handball forward whilst you are standing still.
(Ironically, this is something that AFL players mostly dont get right, even today).

When guarding on the mark to prevent a handball over the mark, keep the hands down but ready to reach up or out to block the handball (the player with the ball will take his "sighting" from the top of your body and if that is your head rather than your hands, it provides the opportunity to shoot your hands in the air and effect the block). When we researched this approach it increased interception four-fold.