What could be done to increase turnout constitutional amendment elections?

Political parties have used state-level ballot initiatives to motivate voter turnout in different ways.
For example, in 2004, the Republican Party put a constitutional amendment on gay marriage on the ballot in many states. This move motivated Republican voters to show up on Election Day to vote against the measures, which defeated the initiatives and helped President George W. Bush beat John Kerry.
In 2018, the trends in ballot initiatives show a focus on transportation and taxes.