(I know, I know -- I need to get back in picture mode! working on it ...)
This article is about copying the spinning of commercial yarn. See an earlier article on dyeing self-patterning yarns.
If you can get your hands on a sample of the commercial yarn you like, look at how many plies it has (sometimes I try to duplicate that, but alot of commercial yarns are crazy numbers of plies, like 6! eek... I think Tofutsies was 4 plies with each ply having 2 plies within it), the wraps per inch, and the twist angle. Granted, they have machinery spinning the fibers and you're human, but mimicing WPI and twist angle/TPI can guide you along the path toward a similar feeling handspun.
You will find that a 3-ply gives a much rounder yarn than a 2-ply, so for mimicry's sake, you might want to make at least a 3-ply to make something closer to the commercial yarn's many plies.
Some commercial yarns that look like singles may actually not be; but don't let that stop you. You can still mimic their wraps per inch and twist angle with handspun singles.
When mimicing commercial singles such as Brown Sheep, I like to take a 6 inch piece of the commercial yarn, tie the ends together so it is folded on itself, and put it in a sink of warm water. This will activate the twist so I can see what my ply-back test should look like when I'm copying the singles.
Doing this warm-water test even with plied yarns lets you know if the commercial yarn was spun to be balanced, overplied or underplied as well -- I have only tested out singles, but commercial yarns are often steam-finished, which sets the twist quite thoroughly.
(expansion of a post by me on knittyspins, 21mar07)