The limits on the longer exposure is movement. Most lenses with image stabilization can be shot down to about 1/60th handheld. The other problem is the snake can move as well. I'd say raising the ISO, depending on how your camera can handle it, is the best way to increase exposure so you can back off on the F stop a bit.
To the OP, the depth of field is incredibly small in these shots, which makes me think you were at a pretty wide open F stop (low number). Not sure what lens you were using, but those photos are akin to using my Nikon 50mm at F1.4 which gives an inch or so depth of field.
The answer is really just getting better light. A white sheet and a desk lamp can do a pretty decent job. I think there's a how-to for making a budget light box as well somewhere on here (which is how most of the breeders take such nice photos).
I also am a Nikon guy, but haven't tried doing any nice snake shots yet. I think the light box is the best way to do it and plan on trying it myself.