I'm knitting up a pair of lacy socks (for me? my Mom? Dunno yet!), using a wool/silk/bamboo blend with no nylon, so I've decided to reinforce the heel using slipped stitches - not only the flap, but the heel "cap" (the bottom part of the heel, that you actually walk on) as well.
The sock pattern is from Knitting Vintage Socks. I like this book - it gives clear instructions for a variety of heel types and I'm using the "Dutch" or "horseshoe" heel here. It has a rectangular heel cap, one that's ideally suited for continuing the slipped sts that make up the flap part. Here's a generic recipe for a flap with a ridged slipped stitch pattern (note that I slip the sts on the WS or purl side, which I prefer - makes the rows go by faster!):
Heel flap over EVEN # sts:
RS: Sl 1 pw wyif, K to 2nd last, sl 1 pw, K last st
WS: Sl 1 pw wyif, [P1 Sl 1] to last st, K last st
Finish with a WS row. The extra slipped st at the end of the RS row makes for a tighter chain edge. I find it really helps.
Turning the heel (making the "cap"):
Divide sts into thirds, adjusting so that the middle portion has an EVEN number of sts, place markers (those split-ring markers come in handy here).
RS: Sl1 pw wyif, K to 1 st before 2nd marker, sl 1, (remove marker) K1, psso, turn.
WS: Sl 1 pw wyif, [P1 Sl 1] to 1 before 1st marker, p2tog (remove marker), turn.
Repeat these 2 rows until all the side sts have been "eaten", ending with a WS row.
You won't need the markers after the first row - you'll be "eating" one st before and after the "gap" at each row. It's obvious once you start.
Some tips: keep the stitches tight when you psso and p2tog. You might want to substitute an ssk for the K1 psso, but my ssk's tend to be a bit looser and hence don't give the best results.
Because the heel cap has a constant number of sts, it's easy to keep up the slipped-stitch pattern for reinforcement. I find it much harder to do if making a "French" heel, where the cap is triangular...and I find the fit pretty much the same.
Once you've picked up the sts on the sides of the flap and are knitting in the round again, you can revert back to straight knitting (stockinette) on the sole. Or, you can continue the slipped-stitch pattern a bit further into the sole; however, keep in mind that this type of pattern works up narrower than stockinette, so the sock will pull in. You might have to adjust the rate of your decreases at the gusset...