Do not think about snake and fall into sleep with the vague picture of snakes (or any thing) but think clearly about snakes (or anything) and close the picture before you sleep. Those that come into dreams are largely the vague pictures and doubtful characters. Those that you think clearly and strongly will not appear in dreams. Another thing, when you are afraid of any thing you see only part of the problem. If you analyze it and see the issue completely it will not come.
For example: if you are afraid of snakes, it is very likely that you do not want to learn much about snakes, you fear to know more. That means you know about snakes partly. Now sit and learn about snakes fully - how they are, how to catch them, how some people live with snakes, and how some carry them around for a living and make them play for the public. If you are afraid of anything, learn fully about them. You will not have any snakes in your dream.
Source(s):
Experience