State policy. Automatic voter registration is the norm in many modern democracies. Rather than forbidding eligible voters from voting until they "opt-in" by registering, eligible voters should be able to vote by default, with the ability to opt-out if they choose to.
State policy.Otherwise-eligible voters should be able to pre-register to vote before they reach voting age. That way they can will automatically be able to vote as soon as they become eligible.
State and local policy. It is easy for high schools - that already have their students' information - to ensure that all of their students are preregistered for voting. That way, more prospective voters are registered as soon as they become eligible to vote.