Weathering Textures - Best way to do it?

I like to create and edit liveries from time to time, but one thing I've always wanted to do but never have done is weathered textures. I don't mind a clean aircraft, but sometimes flying something 'dirty' can really bring the simulator to life. ;)

In the case of editing a texture, what would you guys recommend me doing? Some people talk about creating a noise layer and using that, but it doesn't really give back great results.

What's the easiet way to do it? And are there any good videos I may have missed when searching for this myself?