If they have children, appeal to their sense of duty as parents. Consider using any argument or tactic, so long as it is not unlibertarian. Appeal to emotion, reason, religious values, business hopes. Point out that there are advantages to being a candidate, such as name recognition, respect, media attention, and advancement within the Libertarian movement. Study your people, figure out where the buttons are, and push them.
Persistence is a virtue.
Hi, it's me again, your friendly neighborhood libertarian chairman, calling to pester you about running for the state legislature. When it's time to get to the filing office, you can't afford no-shows. I will also admit to appealing to the vanity of the person recruited. Jane, we simply don't have anyone else. You are our last and best hope.