I'm not completely against trying to sell penalties. Referees have a tendency to be on the stupider side of life, and exagerating an incident where a call should be made either way (Like running into the kicker, where its suppose to be a penalty regardless, but kickers tend to sell it so the ref doesn't miss it).
The problem with soccer is not only do these guys exaggerate contact, that fall to the ground and roll around. If you've ever seen anybody get a REAL injury, they don't squirm around very often, and when they do, it isn't for very long. Rolling around on an "injured leg" doesn't help it. I'd be totally cool with a soccer player playing something up that was a blatant foul in the first place just to make sure its noticed, but for the sake of their dignity, they need to learn to simply fall to the ground or work with the foul. Most punters just make sure they appear unbalanced after a hit and fall down. Hockey players get right back up again and keep going, because if they exaggerate too much, they get called to. If you're going to sell a tripping call (Hopefully a legitimate one), you just trip, get your butt up, and keep playing whether you got the call or not.
I suppose my point here is "Sell, don't flop".