Have another responsible person there so they can get your snake when they take off. I'm able to photograph very fast garter snakes this way and while obviously it's fine if they take off, my helpers (older kids) never let that happen.
Also, don't try to take pictures by a spot that they can escape and disappear. We might use a short tree, surrounded by grass, that is not near any other tree. If we use a rock, we make sure there are no holes nearby.
The picture I attached is from May 2014. This snake always likes to go down, so we place him a little high and then let him work his way to the ground. My daughter is just outside the frame, ready to rush in if he doesn't anything unpredictable! When snakes are outside, they are curious about the new surroundings, and a lot of times even fast snakes slow down and tongue flick for quite awhile, wondering where in the world they are.