Although I love "afterthought" heels, I felt like there was a way I could make them fit my incongruously small heels just a little bit better. The heels on the other socks I made fit pretty well, but I thought I could do better. So I just left out three rounds of plain knitting, and voila! No sag at all now.
My "Canyon Sock" in progress, with heel done and starting the rest of the foot:
Previously, I had done the heel as follows: Knit three rounds plain, knit a decrease round (six decreases), repeat until I had half the stitches I started with, and then decrease every round until I was down to six little stitches, draw the yarn through the loops, and it's done.
This time, instead of waiting till I got to 36 sts (half of my original 72), I started decreasing every round when I got to 42 sts.