there are 3 ways i use to get around shading skins:

• texturing
  
• rounded shapes
• using layers

for your skins to improve the best options i can see is to reduce the contrast on the skin by avoiding pure whites and blacks, avoid 1 pixel wide lines and use the layers to add depth

Shading is key to making your skin look good... I recommend using a template for a start and then try doing one yourself :) Hope I halped you!

So, if you do not want to use any shading and you are entirely opposed to it just try to fill in the empty space; add lots of detail (try to not make it too overwhelming on the eyes), however, without shading you are missing out on a lot of detail and you will only be able to reach a certain point until you start trying to shade.

add simple details to it.

Hmmm...

That will be hard. Shading, (I know you want without) is in my opinion a very important part of the skin...

However, there are some skins that look nice even without shading. I'm not doing them because it's not my style, but I saw some. Everything depends on how you present the thing, the details you put and the colors.

getting the features on the face right is an obvious plus
"without shading" is kind of a no-go since flat colors will look very ugly without any kind of noise or something added to them

the approach you take depends on the art style you're actually going for