He became a professor of engineering.
"I am, and will ever be, a white socks, pocket-protector, nerdy engineer" - Neil Armstrong
He knew that staying at NASA would mean being a figurehead behind a desk. He was not just a hot Right Stuff flyer but also a gifted mind in his engineering field, so he switched to doing something of practical use: teaching others. Once in an interview mentioned that he felt someone should not be measured just by one big moment in the spotlight but by the sum of all that he did to get there and what he did with it afterwards. Had the composure to realize that having worked and risked mightily as a part of the team for this achievement, he had the right to live life on his terms rather than be sucked into the celebrity spiral. Very minimal appearances, only for causes or businesses that he believed in (e.g. when Chrysler needed its first bailout back in the late 70s), no autographs after a point, no reccommendations for people he did not really know. But on his rare appearances always came across as a warm, modest person, with that quiet confidence that comes from knowing you don't need to prove anything to anyone. Not a ham like Buzz but not a recluse either. Just a gentleman and a gentle man who believed that we have the ability to solve problems and achieve great goals if we put our minds to it.
Once when asked about walking on the moon he said that a pilot feels best when he's flying, not when he's walking; and that one thing he found remarkable was that he never dreamed about being on the Moon... What need to dream, sir?
@pwhodges: yes, others have commented on the quietness of the reaction, but looking at it, it just fits the man, doesn't it? He achieved many great things, rejected the cult of celebrity and had a long, productive, good life well lived and was loved by those close to him and admired by the public to the end, and we knew his time was growing shorter already. So like his crewmates we are sad he will not be there for the 50th anniversary or to congratulate the next human on another world, but we'll give that wink to the Moon in his memory.
If people were rending their garments and pouring Tang on their hair in the middle of the street blocking traffic in Public Display of Grief that would be kind of unbecoming to his dignity. Leave that sort of thing for tabloid-fodder princesses and Pop kings. Our media has conditioned us to that.