Weathering Model Aircraft
There are portions of model building that are very mechanical -- "insert peg A into hole B and add glue". Other portions of model building are more of a routine -- "put paint in an air brush jar, thin it to proper consistency, and paint the model". Following the completion of all the mechanical and routine steps, you have a sterile representation of what the subject would look like if you placed it into a dark vacuum chamber and never exposed it to any elements.
Weathering is a term associated to the process of making the model look like it has been used. This usage takes many forms: dirt and grime accumulations, exhaust staining, sun fading of the paint, rusted metal, and lots more. Weathering is really more artistic than mechanical or routine. I have learned and refined much of what I do today from a guy in my model club that works in graphic arts. It is this artist that built some of the best weathering examples I have ever seen.
While I fortunately have been blessed with some artistic ability, not everyone is. That does not mean that people with less artistic ability can not weather models. It just means they need to work with some simple techniques to start developing their artistic abilities. There is no "silver bullet" when it comes to weathering. More than any other process in modeling, weathering is one that requires practice to develop a set of techniques that work for you.