From a 1970s Peace Corps manual (via outstanding travel writer Robert Young Pelton), "How to survive an anaconda attack."
1. Do not run. The snake is faster than you are.
2. Lie flat on the ground, put your arms tight against your sides and your legs tight against each other.
3. Tuck your chin in.
4. The snake will being to nudge and climb over your body.
5. Do not panic.
6. The snake will begin to swallow your feet first.
7. You must lie perfectly still. This will take a long time.
8. When the snake has reached your knees, reach down, take your knife, slide it into the side of the snake's mouth between the edge of its mouth and your leg. Quickly rip upward, severing the snake's head.
9. Be sure you have your knife.
10. Be sure your knife is sharp.