I'm too busy trying to get women in their late 40s naked to convince someone to try Linux.
Seriously, though, they have to want to (or at least be open to) change. You can give your best performance yet, and if they aren't interested, they won't go. Now, every once in awhile you run into some person who is interested in trying new things and learning more about computers that has somehow never heard of Apple or Linux. Apparently, JJ has found one.
Anyway, there aren't that many people open to discussion. So you just drop little hints about not having antivirus software, or free graphics programs, or monopoly practices, and see if they go for the bait. If they bite at it, and are interested in hearing more, then bring out your big guns. Otherwise, leave it alone, because you're just going to get frustrated.