Random observations concerning glyph design:

1) A traditional symbol trumps logic. Even though a snake-entwined stick might not make sense as the symbol for medicine, it's too late to change it now. The same goes for concepts like barber, birth, cold, horror, save, shark and theater. Without years of conditioning, you might never make the connection between the symbol and the actual word, but since we've already been conditioned, we might as well stick with it.
2) There should be something that clearly distinguishes each glyph from other, similar glyphs, even when they are drawn hastily or sloppily. For example, the profile of a horse is clearly differentiated from the profile of a deer by the antlers. No matter how badly you mess up that horse, no one is going to mistake it for a deer without the antlers -- and vice versa. On the other hand, the profile of a bear and a pig have nothing specific that differentiates them. Even if they don't look anything alike when carefully drawn, a hasty scribble could be seen as either. Maybe the pig's curly tail will be enough to tell them apart, but it might be more distinctive to draw one as a face rather than as a profile.
3) Sometimes, long established tradition will challenge Observation #2. For example the Ankh and Venus are sort of alike: a loop atop a cross. In cases like this, we'll have to stick to tradition, and risk the possible confusion of symbols. The same goes for the Dollar and the Staff of Asclepius; they might look too similar when hand-drawn, but the tradition is already established.
4) There's no rule that says that the difference can't be something really, really basic, like which way the glyph faces. It can be something as subtle as whether the feet point apart or in the same direction. Remember -- in alphabetic systems, the difference can be as little as p versus q versus b versus d. As long as the differences are standardized and obvious, people can learn them.
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5) Trust me: sometimes the glyphs are easier to draw than they appear at first. For example, it took me forever to figure out how to draw a recognizable rose in black and white. Even a standardized heraldic rose was too complicated. Then I discovered that if I kept drawing pentagons, one inside the other, I got my rose.
5) There's a trend in graphic design to favor isometric projection (looking at it from an angle) instead of orthographic projection (looking at it dead on). Giving a slight tilt will help illustrate a cylinder, a hat or a coin. Unfortunately isometric is always harder to draw than orthographic, without necessarily being more useful. Drawing a car, a house, or a printer from an angle makes the image more complicated without adding any necessary information. I'd suggest avoiding isometric unless it significantly clarifies the image.
 Orthographic Isometric