I don't have any tutorials, but if I want to paint something like an eagle or skull, I first draw it a couple of times on paper. Untill I know how to get it just right (like you could almost draw it with your eyes closed;)) and then I'll try it on the miniature, with very thinned paints.
In Boltmans tutorial on Magmatrax he desribed how he did the skull on the powerfist, it's almost the smae way I do it. maybe it's worth reading that part again