Each election a portion of registered voters do not exercise their civic duty. Of the 146 million people registered to vote at the time of the November 2008 election, only 131 million actually voted.
Each month the U.S. Census Bureau surveys households for the Bureau of Labor Statistics. The results of this comprehensive survey, called the Current Population Survey (CPS), provides among other things the official unemployment rate for the country. During election years, the CPS includes a Voting and Registration Supplement to document voter and non-voter trends and characteristics. The November 2008 CPS results gave insight on why 15 million registered voters did not vote. The majority of reasons given could have been solved by voting via absentee ballot.
Absentee voting means instead of voting at a polling place, you fill out a ballot at home and then mail it in after having your signature witnessed or notarized. Check with your state or county election board for absentee voting requirements.