What kind of camera are you shooting with?
As for keeping the gartersnake still, try holding your hand flat over the top of the snake. Make sure the snake is coiled under your hand, and press down lightly on the snake. Hopefully this will cause the snake to relax as it feels like it's under tight ground cover. Some people place their hat over the snake, some place a bowl! I found the hand trick to work the best.
Now once you can get your snake to sit still (which usually isn't very long), try to make slow movements, and have your camera turned on and ready to shoot at his point. When you focus on your subject, try to get the eyes in focus, I feel that's key to a quality picture. As for compostion and possing the snake, that is left up to artistic interpretation. There's a few guide lines that I like to use while setting up a (snake) shot.
1: Make sure the entire snake is in the picture for full body shots! Don't take a picture of the entire snake, just to have the tail or part of the body half way out of the frame. For close up shots, you can as much, or as little of the snake in the frame.
2: Try to keep the head elevated or resting ontop the body. Letting the head droop or lay on the ground, gives the animal a sick or dead apperance.
3: For the phtography buffs, you use the "rule of thirds". Basically this means, you keep the aread of focus off center. Typically the areas of main focus would be in the mid-upper right, mid-lower right, mid-upper left, and mid-lower right sections of the frame. I'm not sure really how to explain this other than drawing a diagram. Just divide the picture into thirds using imaginary lines, vertically and horrizontally, where the imaginary lines intersect is where these points of focus should be.
4: Try to keep the snake's face off the edge, or looking closely at the edge of the frame. Try to position the snakes face that it's looking across the picture, rather than looking like it wants to leave. Also, go for a profile shot of the head, rather than a head on shot. You'll have better success of having the eyes in focus while shooting profile shots.
I have a ton more pointers, but I think that's a good start. Basically, have fun and take LOTS of pictures. The more pictures you take, the better the chance you have of there being a really great one!
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