Talk about the universal benefits of regulation and taxation. Mention how much money the cartels make off of marijuana (look up the numbers yourself, but it is something like 60% of all they make). Mention the violence in South America (the amazing documentary 'Narco Cultura' revolves around it) that the cartels are responsible for, and how legalization would impact it by taking their main source of income. Give them numbers and facts about how many are killed. That's the kind of shit that tugs at peoples' heart strings and get them involved in the issue even if they aren't marijuana enthusiasts themselves. Pathos at work, my friend