Compared with other nations the US voter turnout is at 139!
Fair Vote has some suggestions.
"Proportional representation, which is a system that better represents the varied choices and beliefs of the electorate, is usually associated with higher turnout in contrast with winner-take-all plurality districts. A National Popular Vote (NPV) with Instant Runoff Voting (IRV) would also better represent the views of members of minority parties. IRV would also reduce negative campaigning with the proliferation of multiple opposition candidates. These both are associated with higher levels of turnout, especially among minorities and young people, in the United States and abroad. Finally, FairVote advocates Universal Voter Registration to make the voting process easier. The barrier of registration in the United States has been estimated to deter voters and decrease turnout by as much as 10 percent. Furthermore, registration barriers that depress turnout contribute to the "socioeconomic skew" of elections in which the wealthy are more likely to turnout than low-income voters. Thus Universal Voter Registration would likely reduce class barriers to voting. There are also a number of turnout boosting institutional changes that are issues not represented by FairVote. ..."