Rigul and Chipp are both on the money of course..different methods for different needs and wants.  

Chipp....highlight the layer and then use the little floppy icon at the bottom of the material editor window. Takes you right to a saving place....

"What I like to do is mix my materials first, save them and then use them in layers....."  What I meant was 'with' layers like Chipp did in the pic....you can't use mixed materials 'as' layers unfortunately.