As far as porportions goes, I always look for the 'scale' of the subject. In each and every individual, there is scale of measurements that makes them look fluid and well built.
For example, with a horse- my main interest. When looking at a picture of any equine, there is a way to measure everything using only one part of it's body as the scale. This scale differs, ofcourse, with ponies and say... thoroughbreds, but any good artist will quickly be able to pick it out. With a horse, the measurement from the poll of a horse's head to the tip of its muzzle will be the same as the width of it's heart girth to withers. It's ears and muzzle will be the same size. The legs (beginning at the shoulder) will be as long as it's entire body from chest to rump, and so forth. Little details like this are what give your picture good symetry, without looking overworked.
I did not know this. I do all my scaling either by eyeballing it, or using a pencil , holding it up to the screen for ratio porportioning.
Another thing I often do when working with very small but important details like eyes. When I look at a certain line, such as the curve of the eyelid, I will really look at the line and try to decide if it is convex or concave, sloping in any direction and where is the most extreme point of the curve. When you do people's faces, the shape of the lines that define their eye can be very minutely convex or concave and if you don't get that, the whole face doesn't look right. I will often put a straight pencil up to the screen to compare a line to it to see if the line is truly straight, convex or concave.