powdered tide and dawn both work. I usually use dawn, just because that's what I happen to have. I change the water once or twice too, and add new soap(not too much or you'll have a foamy mess) but it seems to help with the grease. As stated above, simmer, don't boil, or things will start falling apart!
I bought a skull cleaning kit from GM. I'm in the process of doing one right now. Just getting the last of the "stuff" out the hard to get to places. I did the same with the first one I did, and it turned out great. The last one I wasn't too picky with so I used Dawn dish detergant, and while it's OK it not great either.
i used a kit from van dykes taxidermy. it has the boil stuff and a powdered peroxide for whitening. it costs about $25 shipped from cabelas or others. you can do at least 3 skulls with it. i highly recommend it. but as others said, simmer, don't boil. and don't use bleach or the bone will break down quicks.
the longest part is skinning it and getting the eyeballs and brains off. need a fillet knife or something really sharp and flexible. you need to get as much as possible off and then simmer it for a few hours. pull it out once in a while and use a knife and little wire brush to get it all off. i would plan on a day from start to finish. and don't let the head fester too long after its dead or you will be presented with a serious mess. i do mine a day or so after butchering.