It's pretty tricky trying to convince other adults about anime and manga. Over the years I've met a few people that I was able to slowly ween into the genre. Usually I try and find an anime to match what they enjoy in live action. For example, a housemate of mine really enjoyed Harry Potter, so I introduced him to Fullmetal Alchemist. after the first few episodes he was hooked and ended up marathoning the entire series over the course of a week. He then begged for more so I hit him with Deathnote
. Really anime is for people that enjoy good story telling and cool visuals, so when I come across people that think Lord of the Rings is "stupid" and that they "don't get it" I dont waste my time. Yet I don't understand some of the stuff they are into, so it's just a case of different strokes for different folks.
As for One Piece on the other hand, I would only try to convince an already existing anime watcher to take a look at it or try to explain it. OP is so rich in story telling and character development, but some of the humour can be missed (perversion not withstanding) it's something that only the fans that understand the characters will get. If you ever find yourself in that awkward possition with a person (particularly a girl) that doesn't watch anime and finds you watching One Piece, try this on them. Compare it to a day time soap like Days of our Lives. People watch it every day religiously because they want to know what happens next to there favourite characters and how the ???story??? progresses. One Piece is the same except more compelling, colourful, and childish at times, where as DooL is more shallow, mature (depending on the psychological state of its characters) and consistent. These are just my words though, based on how I view humans. Seriously though, if someone has a go at you for being childish for watching cartoons, I'm pretty certain you could find a way into their heads and mess with them, bringing up some sort of psychological damage, reveal their weakness and batter them back into place. This is a last resort though, it's good to be good