Some of the sexiest people I've met, haven't usually been conventionally pretty or good looking.
I can't usually even put my finger on what it is that makes them sexy. As pp have said, you either have it or you don't.
If I really think about it, I suspect it's something to do with confidence, but subtle, not ott or in your face and also, assertiveness, intelligence and being really kind. All those things together, I would say I find very sexy.
I always thought I was strange, because I can't look at a person and be attracted to them without knowing them.
That said, if you're all of the above and you look like Gillian Anderson or Aidan Turner....well 
My DW has all of those things though, plus the gorgeous exterior, so Gillian and Aidan who?...... 