I’ve been reading a bit about reinforcing heels and toes on socks, and one reference I found advised to knit the thread that came along with some type of sock yarns to reinforce toes and heels. Since I’ve never seen thread with sock yarn, I was wondering what you all do. Just double up the yarn for heelsand toes? Sewing thread? Thanks for sharing! 
Reinforcing heels
Don’t double up the yarn, it will make the heel too bulky and uncomfortable in your shoes. You can go pick up some nylon thread at a craft store, or check your LYS for reinforcing thread. Just knit it with your sock yarn holding the two strands together.
Another option is to knit the heel flap using a K1, S1 pattern. It bulks up the heel just a bit so that it’s sturdier, but you don’t have to use a separate strand. Check out my sock knitting tutorial for this technique.