Having grown up in the woods of the Deep South, snake encounters were frequent. They would often be found sunning themselves of rocks in the spring or fall, or cooling off under logs on the summer. It can startle you to suddenly come upon one of these legless creatures. As a biologist, I learned to deal with them.
What to do when you encounter snakes in the wild? First, take the advice from the cover of Douglas Adam's Hitchhiker's Guide to the Galaxy when dealing with unfamiliar situations, "Don't PANIC"! Remember over 95% are non-poisonous and those 5% are usually poisonous enough to kill you. Contrary to popular belief and the movies, they can only jump 1/3 their body length, cannot really slither that fast (they usually just slowly glide over the ground or through the water, it looks faster than it is) and are more afraid of you than you are of them. You are much bigger and might step on them!
So, if you see a snake, what should you actually do, besides not panicking? Stop where you are and carefully assess the situation. By running or jumping, one can easily twist an ankle or break a leg; you are then on the ground with a frightened snake, which if it chooses to, can reach you much easier. You should also make sure there are no mates nearby. This also gives the snake time to find ways to escape, remember it is trying to avoid you! Stopping and thinking are crucial, act rather than react.
Now that you have looked things over, slowly back up (unless it is that Giant Anaconda from South America that one sees in the movies, then you might move a bit quicker). If you are in a clear area, go around the snake, keeping it a body length away from you. If in the woods, if you are in a hurry, you get a very long stick and carefully nudge the snake out of your way. The snake may strike the end of the stick, but no harm done. Do not beat the poor animal; you are in its home, where it is supposed to be and it is merely protecting itself!
Once you are past the snake, ignore it as (unless it is that Giant Anaconda from the Brazilian Rain Forest) it is not going to chase you. Snakes will only attack for food (most people are too big for that) or when they are threatened, once past you are leaving and the snake is probably heading for cover. Many people worry the most about Rattlesnakes, but they are rattling to get you to go away!
In summary, when encountering a snake in the wild remember these few basic things. Stop, look, and think, but above all don't panic. Either go around the snake or use a long stick to gently push the snake out of the way. Never threaten or try to trap it as threatened, trapped animals become defensive animals, and may bite. Then continue on, enjoy your hike, you have just had an encounter with a fellow creature that also enjoys the outdoors!