Since Guillermo was also a creature FX guy at one point, he also gave me a tip for using foam latex, should I ever get any and upgrade my costume. I figured I'd pass it along to you guys. From the man himself:
We used UPHOLSTRY FOAM for some large tentacles and in order to "fine texture" them we used to mix -from a foam latex kit ordered from BURMAN FOAM- A measure of Latex base and half a measure of "gelling agent" (the grey stuff) and use a loose sponge to "sponge" the surface wet.
We then let it dry and then "tool it" with some wooden sculpting tools wetted in light, sopay water. That would give your sammy a lot of "wrinkle and pore" life.