For a long time, I solved this problem by picking up the stitch as usual, but then knitting it through the back loop, to make it twist and tighten up that line. It looked great on the side of the sock BEFORE the instep, but not so great on the side AFTER the instep (we'll call this "Side B"). I finally figured out that this is because, on the side after the instep, the stitch is "pinched at the bottom" on the right instead of on the left.
The solution? Twist the Side B stitches, but in the opposite direction. This means using the left-hand needle to pick up the slipped stitch from the back, and then using the right-hand needle to knit that stitch through the front loop. Viola! Gorgeous results.
I hope to get some clear photos and a video up soon to illustrate. But for now, I'm just enjoying how beautiful my socks look!
UPDATE April 7, 2015:
Well, here is an example of how knitters continue to learn and grow throughout their crafty life! I was twisting my picked-up loops as described above largely because my slipped stitches at the ends of the rows of the heel flap were so loose. As I got better at heel flaps, they were less loose, and twisting the loops became more difficult (and in some cases nearly impossible).
But, even with the tighter slipped stitches, I still didn't like the look of my picked up stitches. So I tried a little experiment - instead of slipping the first stitch of each row purlwise, I slipped it knitwise. This twists the slipped stitch and keeps it from stealing yarn from neighboring stitches during the pick-up process. I have been very satisfied with the result, and it is SO much easier than twisting the loops.
Of course, like anything in knitting, you should find a method that works for YOU, that gives YOU a finished object that YOU enjoy. I hope I've been helpful in your knitting journey!