The whole key to skin succesfull is to look, not only look at the model, but look at the world around you, study different materials and make sure you understand how it reflects on serval objects.
Dodge and burn tools are your friend, use em wisely because they can make or break a model. If your done with the skinning part then apply a sharpening filter and as a final touch up the contrast and change the lightning up to how it suits you.
here are a few guidelines i use for different weapons wich should give you some insight.
metal: it is reflective, but not quite as much as chrome, usualy varying from a dark gray to a white basic colour but can be adjusted with heu saturation. Metal scratches easily and bounces the light around the skin. Skin in some scrachtes with dodge on highlights, 1PX and 100%.
Chrome: Most of the times chrome looks quite clean, it reflects almost all of the light. Usualy seen as a light colour, along the lines of white, light gray and light yellow(gold look). The base of the textures is mostly the same as metal only less worn. Also the contrast is usualy higher.
Plastic: low level of reflection, shoft shadows and soft lightning. Probably the most difficult material of all to skin. Small scratches usualy show the worn use of it. But, dont overdo it or it will look like metal.
That are my guidelines that i use by skinning.
And why dont you show us a example of what you dont understand, i might be able to shed a better light on it.
Offcourse, the only thing i can say is practice a lot.