There are 2 popular reinforcement options:
1. "beilaufgarn" - use a tough (typically thin nylon blend) thread together with your yarn to add strength to areas of high abrasion. Lang Jawoll includes a small spool of thread dyed to match for just this purpose and sometimes you can buy neutral colours in spools from yarn stores. You can, in a pinch, use "wooly nylon" thread meant for sergers, or, even better, buy a spool of polyester (not cotton!) sewing thread in a matching colour. Just hold your chosen beilaufgarn together with your yarn and knit as one. Beilaufgarn is knit in all around the sock, so it's usually limited to heels and toes (and not, for example, if it is required in patches at the ball of the foot, or in the middle of the sole somewhere).
2. slipped stitches - as you knit the heel flap, slip every other st on either the RS or the WS. Knit (or purl) the other side as per usual. If the sl sts line up, you get attractive ridges. If they don't line up (ie. you alternate sl 1 k1 on one row with k1 sl1 on the next k row) you get a honeycombish pattern known as "eye of the partridge". Either way, the resulting knit fabric is denser than usual and stands up well to abrasion. Typically, patterns call for slipping the sts on the knit side (or RS) of the work, and purling all the way back. I like to do it the other way, sl 1 p1 on the backside and knitting all the way back on the front. This makes my purl rows go faster! You can also use the linen stitch, which uses slipped stitches on both RS and WS.
Traditionally, only the heel flap is treated in this way. But there is no reason why you can't reinforce, in advance, any spot that you know will wear faster.
For myself, I need to reinforce the toes (toenail side), so this can be done by knitting slipped stitches there. My mom wears out her socks aournd the balls of her feet, or under the heel - this is a "patch" problem and I prefer darning in beilaufgarn after the sock is complete so it is almost invisible - but darning, as we all know, is kind of a pain. Slipped sts can be "applied" anywhere, but are quite visible as the texture of the stitch is quite different from stockinette. They pull in and make the work narrower where used.
If wear is really an issue, consider the Old World technique of a separately knit sole and top of the foot. You can use a hard-wearing fiber for the sole, heel, and toe (ie. a high-nylon blend). You can also then reknit the whole thing if you do get holes, later. Favourite Socks contains a number of sock models that use this technique. The technique basically involves knitting the top and sole of the foot flat, and then either sewing the two halves together, or knitting them together by picking up sts at the edges. I have used it for alpaca socks where the heel, sole, and toes of the piece have to be of a different type of fiber.