I like snug-fitting socks. This means they have to fit properly.
There are many things to watch out for when fitting socks. Here are some that I know about:
1. the obvious: they need to have the proper circumference! Socks are basically stretchy tubes, and the circumference of the sock should be a little less than the circumference of your foot. For me, that means that if I take 90% of my foot circumference measurement at the ball of my foot, and I make the sock this circumference, it fits well around my foot and the lower part of my leg (to about 8-10 cm above my ankle bone). If you are doing toe-up socks, a guideline is to stop increasing when the toe of the sock fits over your first 4 toes (not including your baby toe, in other words). Most socks that use fingering weight yarn (400m/100g) call for approximately 60-64 sts around for women's socks (medium) and about 72-76 sts around for men's (medium). If you use sport-weight yarn, these numbers will be smaller. If you use very thin yarn (Regia 3-ply), or are knitting cables, you will require more stitches. Usually you cast on multiples of 4 stitches.
2. second obvious: they need to be the right length in the foot! This one is a little trickier. It is easier to fit top-down, flap-heel socks for proper length: after turning the heel you continue knitting until the sock just covers your little toe, then start decreasing for the toe. If you are knitting toe-up, it is harder to judge where to start the heel construction / short row / gusset, because the heel construction itself adds length (especially for short-row heels). If your socks are too short in the foot, you may find that the heel gets pulled down and the sock slowly slides into your shoe over the course of the day.
3. less obvious: the heel flap needs to be long enough. If your socks wrinkle at the instep, the flap is too long (like with a tube sock). If the sock is too tight across the instep, your flap isn't long enough. (What tends to happen with socks like this is that they slide down to where they will fit - futher down your foot. In other words, they slide into your shoe.) Note that this fitting rule is true even if you knit a short-row heel, one without an obvious flap! In flap-style constructions, this depth is easy to adjust - you just knit back and forth on the flap for as long as you need. In short-row constructions, adjustment isn't so obvious. A short row heel's flap height is the same as the "depth of the heel", and this is set by the number of short-rows you do.The easiest way to increase the length of the back flap is to increase the number of stitches that you use for the heel. You can do this by either using up some of the front-of-foot stitches (which is what Cat Bordhi does for her STH), or, you can just increase stitches just before starting your heel, on the heel needle only. This will make a deeper heel cup = longer flap. It will also add length to the foot.This means you sometimes have to try several times before "getting it right".
4. Longer socks need to be big enough at the top of the leg. If the sock is too tight at the top it will fall down, quickly. Essentially the sock goes to where it will fit, which is lower down your leg, if it is tight. The way to keep longer socks up is to make them fit all the way up your leg, shaping them from the ankle bone upwards, especially if you are not going to make them go over your calf. If you take them over your calf, then you can cinch them at the knee (traditionally done with a garter) and they won't fall down no matter what.
While all of this sounds really complicated, I've found a few "recipes" that work well for me and I stick to them. They are:
1. forethought heels (including where to start the heel), as detailed here
2. adding 20% stitches to a short-row heel, as detailed here
3. top-down, flap-style socks (I find these are much more "fool-proof" in terms of fit, but are a bit more complicated to knit) with 2.5" of flap for ladies' socks and more for men. Don't stint on the flap length!
I recently had the privilege to review a fellow sock-enthusiast's sock knitting recipe - she calls it the "fish lips kiss heel" - and this is excellent as well. I've only tested this fitting technique in toe-up mode, but will have to provide a review here sometime soon!