what do you want to obtain ? isolated bones, like the elements of a puzzle ? http://www.acanthoweb.fr/?q=content/des-os-la-pelle ? http://www.acanthoweb.fr/?q=en/content/multiple-bones-bluefin-tuna-head
if so, there is no real protocol, as each species presents differences in osteology, myology, skeleton composition ...
to answer quickly, the simplest (quickest?) solutions are:
1. to let decay in water. The duration of the process ( from several days to few weeks) depends on size, composition, initial stat .... Be ready to smell differents things in the room!
2. to boil the whole head in hot water (few minutes) and dissect bones afterwards
3. if the head is not too large (you cannot do it on a head of a swordfish!), you can place the head in a plastic bag with few water and trypsin. Then, you put this plastic bag in microwave oven for few minutes.
Anyway, flesh won't disappear (first one excepted!), they will only become easier to take off ! Those technics are only helps for dissection and, as all fish heads are different, it is different for quite every head. Just try !