I usually make a little Sculpey model of the head... actually, i made them about 8 years ago and just use them as a reference now. That helps me tremendously when making the full-size foam version.
Then, I take 1/2 inch foam and make the basic shapes, using wedges and darts to make a full-size head. I even glue in a makeshift mouthplate out of neoprene. Then, I look at the "finished" mock up, with all it's sloppiness, and cut it down the middle into two mirror halves. I take the better of the two halves, cut apart all the darts where I can and trace the flattened foam piece onto a piece of sturdy paper. I can usually get the flattened half into 1 or two pieces for the final pattern. I do the same with the half mouthplate.
Then, all I have to do is trace the pattern twice onto foam, and trace the mouthplate twice onto whatever the final material is (gasket rubber, plastic, etc.), flipping it on the cut to make it symmetrical, and voila! A symmetrical puppet. I can even use the discarded "intact" half as a guide in putting it back together.
Now, the beauty of doing it this way is that it's now a symmetrical puppet -- and more or less repeatable since you have a pattern. So, when you do the "skin," you can pin a similar fabric directly to the final skull, only to where the two halves join, putting in all the darts, etc. Then you can cut away the darts and the outside edge of the low-grade fabric, and use that as a pattern twice on the final fabric.
It's a little disheartening to do so much work initially with the prototype head, but I like that I only have to do it once. I've made several puppets off the same pattern, changing features and whatnot, but never having to adjust the patterns one bit. For instance, all three of these puppets use the same exact head pattern:
http://www.melonpool.com/BLC/ralphdone.jpg
http://www.melonpool.com/puppets/Bob.jpg
http://www.melonpool.com/puppets/Hildy.jpg
In the long run, I think it saves a lot of time.
Steve