From my years of tag (signature making) flow can be crucial for a piece of artwork, but it is always not needed. I started with no flow in my tags and needless to say it was just a clutter of pixels.
For those of you who are wondering what flow is, it is simply the direction the effects and such go in the tag. Here's an example with an older tag of mine:
Notice how all the effects start from the bottom right corner and work up to the top left corner, this works well as the render is set at an angle.
Here is one of my older tags without no flow:
These effects were just slapped on and really don't go with the render at all in terms of direction. I try to stay away from just making a tag with someone's head. Using a image with shoulders and that exposed are always helpful not to just you but your critics as well.
Depth is something that I find critical in a tag, because without it your tag will seem flat. Now Depth can be added into a sig many of ways, but here are the ones that I always use:
The blur tool, make it your friend. Blur parts of your tag that are not the main focus in the sig.
Black and White gradient map with the layer set on multiply. After you do that fool around with the opacity until you get something not to dark. While the black and white can be useful I sometimes use the orange and violet gradient map set on soft light or hard light and mess with the opacity.
Burn tool and dodge tool. Use them to brighten/darken spots in your sig for some variety. I rarely use this though. Ha
Here's a tag of mine which shows some depth:
I have spots blurred out in here and helps out I believe I always used the black/white gradient technique I mentioned above. And when I did this tag I tried to make it look as if he was jumping at you so you can see farther back is more blurred as opposed to by the render.
Focal point is pretty much self explanatory, this is the main subject in your tag. Now there is alot you can do to make it stand out.
Sharpen your focal point (which is a render most of the time).
Blur unneeded areas that stand out.
Up the saturation on your render.
Use lighting in your tags. (a tutorial on lighting can be found in this thread: http://www.mmaforum.com/getting-star...tutorials.html )
Use unique effects on your focal, for example if you're using a render you can add an outer glow to it or even try to give it a nice border around it.
Here's an example of me defining my focal point:
I blurred areas, sharpened the focal and added lighting to draw your attention more to the render.
Any questions feel free to ask.