I am against layering proper fragrances. I feel that the scent should stand on it's own as an artistic statement. If it doesn't strike me as complete, I don't want it.
That said, if one is going to layer, I would follow what Jenson has mentioned and trace a prominent basenote back to an essential oil and maybe boost the scent that way.
Certain lines claim they were meant to layer. I know Creed has blends that they give their blessing to and I think Bond 9 will do this at their locations as well. I have a strong feeling that these scents were not developed for this purpose, it's just a way to get one to buy more juice and make more money for the company.
I will admit that some of the Creed blends are quite nice.