I have been asked this questions a couple times now since I have macro students who were my micro students last semester. I made sure to put the best case forward for them to save the time they would have spent voting doing something else. However, not voting doesn't satisfy that desire you've been schooled in to affect policy. So how can an "average" person affect policy without wasting his/her time at the voting booth?
More specifically, lie to pollsters. When they call asking if you are a registered voter and what policies you favor, lie and say you are 100% certain you'll vote in the upcoming election. The most prominent national polls will be based on a telephone survey of about 1,000 people. Politicians and their aides read them and extrapolate. You will never be more likely to be the median voter than in that poll.