Yeah, also upfront. Make it absolutely clear, from date number 1, you're after a bf. That weeds them out, and those who think they can lie and get you in bed will also soon be weeded out if you just keep to that demand. It's trite, but if you respect yourself, it shines through, and you'll find a man to respect you just as much.
Settling for a FWB is, I think (unless you are completely on board with just casual sex. in which case power to you), a sign of poor self esteem. You think badly of yourself, and you think "Maybe I can get him to respect me enough to be my bf if I just give him what he wants". That leads to just him abusing you.
Seriously, I think FWB is quite close to rape in these cases. You're manipulated into giving sex that you don't really want on a vague promise that he won't keep at all. Sounds like coercion, sounds like rape.