To summarize, a "good" family" and education helps, as does dress-sense, a tall and slim physique and - even more - a clean-cut face and apparent intelligence. There are, however, clowns from old and "good" families, intelligent, good-looking men who do not, or not quite, make it, as there are well-dressed yobs. At the end of the day, I don't have an answer to my own question.Here.
Edited to add:
Interesting discussion there!