Many newbies and casual 3d artists have a habit of rendering things in one go. This can be one of the reasons that their 3d works seem kind of flat or just lack that edge they look for. Compositing is a skill which anyone 3d artists can learn and should consider when making their 3d art. It's not difficult and can give you alot of control of your final image.

I'm not going to go in depth into compositing but I can give you a simple tip in regards to compositing your sub-surface scattering layer.

There are plugins in many 3d programs these days which allows for Sub-Surface Scattering for things like skin and other natural elements. In some cases, it involves a plugin of some kind and no option to render a separate pass for SSS. This can be frustrating if you want to composite your image in post.

In this tutorial, I'm going to show you a quick and cheesy way of doing this. While I will be using Lightwave (and a free plugin called TB Fake skin), it is not too difficult to transfer the technique to another program.

1) In modeller, load up your model which will be getting the SSS treatment. It is most likely that you already have it textured and everything. Make a copy of this model and make all the surfaces into the one surface as it will be easier for your rendering.

Save it out under a different name, preferably under an easy to remember name. EG if your original model name is "head.lwo", name your single surface model "head_alpha.lwo"

2) Make your scene as you usually would with your normal model then make a copy of that scene and save it out under a new name. EG if your original scene name is "head_scene.lwo", name your new scene "head_scene_alpha.lwo"