Personally I've never had the 'just friends' thing work out with anyone I've actually been with. It does seem to have worked well for my current partner and her ex, but theirs was a mutual decision to break up, so I guess that's a little different.
When I've been 'just friends' with somebody I'm romantically interested in and there's no prospect of it developing further, I've tended to invest all my emotional energy in that person regardless, which is quite unhelpful for finding more satisfying relationships!
In recent years I've adopted an approach where I essentially say "you're great, yes I enjoy spending time with you and I'd love to continue to have a relationship with you, but I've got enough friends as it is, if that's all you want to be." Not sure what others think about that, but for me it's been that simple.
