Three statisticians, Andrew Gelman, Aaron Edlin, and Nate Silver, have attempted to calculate the odds that a single voter would change the outcome of the U.S. presidential election this year. They find that a New Mexico voter has the best chance. A New Mexico voter has a 1 in 6,100,000 chance of determining who wins the election. By contrast, in 34 states, a single voter has worse odds than 1 in 100,000,000 of determining the outcome. D.C. voters have the worst odds; a D.C. voter only has a one in 490,000,000,000 chance of determining the outcome. See this AP story for more.
As an individual voter, your job is not to decide the outcome of the election. Your job is to express support for your candidate, and thereby make your candidate look a little bit better in the percentages.