Every state in the union has launched petitions for peaceful secession, most with little support. Despite being addressed in the declaration of independence, the only current option for secession is through violence.
One option would through a special election requiring 67% of the vote in favor of secession. Such a number would be extremely unlikely to succeed, but simply having a peaceful option to secede would likely prevent a violent one. Such a number likely would have prevented several confederate states from seceding, favoring the union, thus reducing the duration and severity of the civil war.
Enough public support to reach 67% would require egregious abuse by the federal government, in which case secession would be justified, and would preferably happen peacefully.