This is a tough one. Human nature can be a real bitch, and I don't think you can ever win against her.
I think the only time that FWB can really truly work is when the friendship is very temporary - like someone you meet on a vacation trip you never expect to see again. I've had a few guys in my life like that - twice when on trips with my parents, and a few trips with friends in college. You meet someone, you click, you spend a little time getting to know them, then you "scratch each others' itches" ... for a week or 10 days or a few weeks - whatever. But you know it has a time limit...
The worst attempts at FWB are when you promote a guy from the "friend zone" to the FWB zone -- the promotion happened because you like the guy, and FWB is almost, by definition, 100% casual no-strings sex. That doesn't happen with friends, because friends develop feelings, and feeling develop into affection and love, while the FWB is just lust (which is GREAT, don't get me wrong). I promoted a friend from high school (great friend, no sex, just one of those "you can talk about anything" type of guys) from the friend zone into the FWB zone in college (on a trip to visit home). It was great from a while, but then the feelings started growing, and he was getting jealous that I enjoyed the sex but didn't want him to be a real boyfriend. Wrecked a very nice friendship for a while. We've recovered a bit, but it is still awkward sometimes.
Friends are friends, lovers are lovers, fuck buddies are fuck buddies - and those statuses really can't change easily. (and by that, I mean that I think that moving a guy from a true, really good friend status to FWB status is actually a downgrade . Good friends are hard to come by, and sometimes hard to keep because we all travel around and relocate so much these days. Finding a guy who's a good fuck is simple, and they're all over the place. The guy you move into the FWB zone might have wanted you for a long time, and you probably thought something like "its only a blowjob or handjob", or maybe "just one quick fuck won't be a problem". Well, for friends, it changes everything. And not usually for the better.