Haven't even tried. There's no point. Everyone already has their opinion of metal (usually based on hearing one band and deciding it's "all screaming" or something) and they'll only change it by themselves. I don't try to get guys into metal either.
All I do is try to introduce people who are already open to metal, to new bands so they're not just listening to Metallica and Iron Maiden all day long.
30/7 King Diamond
1-2/8 BiesenRock Festival
3/8 Lokerse Festival
16/9 Richie Kotzen