Have the NPCs play to the PCs’ concepts, including strengths and weaknesses. This is an offshoot of making use of background flags, but it stands pretty well on its own. If one of your players builds a character who’s all about being big and tough, have NPCs comment on their stature, seem slightly intimidated in their presence and (once the PCs are well known) recognize them from stories they’ve heard about their martial prowess. In other words, reinforce what your players think is cool about their characters.
I think too many GMs do the exact opposite. They try to hold the PCs back, rub away what makes them special, and try to bury them in minutiae, rather than letting them shine. The ability to challenge your players while allowing the PCs to be and look heroic is, I think, among the skills that separate the decent GMs from the great ones.
Read it all, of course.