Use a plastic knife! Simple as that (and works great for brownies too!). Plastic knives won't cut through the avocado peel, but slice perfectly through the
Slice a ripe avocado in half (you might want to use a regular knife for this).
Holding an avocado half in your hand, use the plastic knife to cut horiztonally and vertically until you have sliced it to desired size. Invert the avocado, and gently press on the exterior of the flesh to release all the dices into a bowl. Voila! Done!