As of 1.8, mob heads of a mob will drop if the respective mob is killed by a charged creeper's explosion.
This is really hard, as (1) finding a charged creeper is rare and (2) luring it near mobs is also difficult and very error prone.
What is the most effective way to get mob heads?
I found a tutorial on the minecraft wiki that seems to be the best way to farm them. It will still take a long time, and it will probably be really hard to build and maintain but if you REALLY want the heads this is probably the most efficient way.
Since Minecraft 1.13 we have Tridents. They can be enchanted with the Channeling enchantment that creates a lightning bolt during a thunderstorm if it hits a mob.
Using it thus gives one a very easy to use way to get mob heads since finding charged creepers is no longer a concern.
