I've been cleaning skulls for about 40 years now, and have tried lots of methods. I currently have about 300 skulls in my collection, after recently donating a few dozen to the local U.S. Fish & Wildlife Service office for their education programs.
Most of the stuff discussed above is accurate. One trick I like is to add about 1/4 to 1/2 cup of sal soda per gallon of water in your simmering pot. You can order it online from taxidermy supply companies, or you can use Arm and Hammer "Super Washing Soda", which is available at many grocery stores. The soda dissolves the meat and loosens up the cartilage and tendons, speeding up the process. Keep checking the progress every 20 minutes or so, though. If you simmer too long, or mix the soda solution too strong, the soda will attack the bone and damage the small parts.
Burying the skull in rich, biologically active topsoil works well, with probably the least work, but it takes several months and leaves the bone brown. The discoloration is easily remedied with the peroxide treatments described above. Remove the brains (to minimize grease issues), and the big chunks of meat and eyeballs (to speed up the process). Dogs (or bears) will dig up your skull and run off with it, or just chew it up and destroy it, if you bury it outside where they can find it. Instead, put your skull in a large tub or bucket, fill the tub with soil (work the soil into the crevices of the skull as you bury it). Put the tub in your garage, up on a shelf where your dog won't get it, and come back in 3 months to have a look. The process is slower if it's cold, so a heated garage works well if you have cold winters. I've never noticed this method producing objectionable odors, but dogs can sure smell what's in there.
Dermestid beetles will do the work for you, too. You can usually collect a few dozen from any roadkill that's been laying along the road for a few weeks. Just dump them into a plastic tub large enough to hold your skull, snap the lid onto the tub, and let the beetles do their work. It usually takes a few weeks, depending on how many bugs you have. You will want to open the lid every few days to give the bugs some fresh air and perhaps moisten the meat so they can eat it more easily. This process does produce strong odors of rotten meat, so you probably won't be using the tub for storing clothes in the future. The peroxide bleaching step will take most of the smell out of your skull once the beetles are done.
