Ribbing for Two-at-a-Time, Top-Down Socks

Once you've cast on, you'll start knitting the ribbing. This is where you have to do a bit of planning, because you cannot adjust the position of the ribbing once you've started (well...you can, but it's a pain). So make sure that whatever you start is symmetric - that your ribbing pattern has a mid-point (either a single stitch, or between two stitches), that is centered over what will be the front of the foot, or you will be in trouble later! Unless, of course, you are making asymmetric socks...but in that case, think about the 2nd one on your needle. Is is supposed to be a mirror image? Or identical? Adjust ribbing - and the leg pattern - as required - before you start!

For example:
k2-p2-k2-...-p2-k2 is good, although you will end up with a k4 band down either side of the sock,
k1-p2-k2-...-p2-k1 is good,
k2-p2-...-k2-p2 is probably not!

Coarser ribbing is stretchier. So, a k2p2 rib is stretchier (and keeps its shape better) than a k1p1. You can try different numbers of k's and p's for extra jazz (k3p2, k1p2k3p2k1, etc etc). You can try substituting a ktbl for every k, to get a tighter effect. It looks a little different as the twist makes the k's pop out a bit more. Don't skimp on the ribbing if you want it to actually hold up your sock.

Alternatively, you can skip the ribbing entirely if you are:
1. making short socks (I am particularly fond of anklets that come no higher than 8 cm/3 inches over the ankle bone.). Try a few rows of garter stitch instead (knit a row, purl a row) to give some "frame" to the design. I like about 3 ribs (that's 6 rows total). It won't hug your leg, but because the sock is short it won't fall down.
2. making lace socks that naturally scallop. Even here though, a small garter edge can add a little "weight" to the design. I find that lace socks are best kept short also, so they don't fall down and spoil the effect.

Advanced notions:

For design buffs, consider having the ribbing flow naturally into the design of the sock. Takes planning!

A nice touch is to have the ribbing flow further down at some parts than in others. Cookie A does this very well in some of her new designs.

Plan for symmetry (or deliberate asymmetry) so that the upcoming division into "heel" and "instep" stitches doesn't wreak havoc with your design!

Try having the pattern only on the front side of the sock. This was traditional in days of yore and can even be interesting today.

For clingy socks that hug your leg, ribbing-based designs are great as they are naturally elastic.

For lacy socks, I find that the best designs are small and based on 6-8 sts, maximum, for an all-over effect. Check stitch dictionaires for inspiration. Of course, all-over and complex designs like those found in German and Austrian dirndl outfits are stunning...

Charlene Schurch's books have excellent stitch libraries that are ideal for socks. Many are rib-based, but she also includes small lacy patterns.

The Burda book has a beautiful collection of dirndl-type designs plus tips on how to knit them into long stockings that go over your calf. You'll have to translate from the German though...