Painful admission: After over 20 years of serious chess, I'm finally, systematically studying the endgame using Fundamental Chess Endings by Muller and Lamprecht.
While I had a working knowledge of the fundamentals, acquired in bits and pieces over the years, and probably played endings about the same as the average B-player, I was doing it with a mishmash of half-remembered positions and pure calculation of variations. I decided a couple of weeks ago that part of making an upward move in the ratings would involve mastering the essentials, at minimum, so thoroughly that I could play them "with my eyes closed"--king and pawn v. king, king and pawn v. king and pawn, rook v. pawn, rook and pawn v. rook, and queen v. pawn, for starters. Also looked in on mating with bishop and knight, which was actually fun to learn and to understand the mechanism involved, even if I never have to use it.
The studying so far has already paid dividends in a couple of blitz games, one where I was a pawn ahead and was able to simplify to a pawn ending knowing it was won and why, and was then able to make the remaining moves almost instantly.
Perhaps the most interesting thing is that just the amount of work I've done so far (maybe 25 of the many hundreds of positions in the book) has produced a new confidence in my play. I've got a long way to go, but I feel that I'm headed in the right direction.
Here's a position that was really surprising to me--Black to play and draw! On the surface, looks like Black's king is totally out of play, doesn't it? I leave the solution as an exercise for the reader. (Or see 2.18 in the book, if you're gonna wimp out).
(Diagram via ChessUp.net)