Panel Shading (or any shading)

I will soon be painting my B-24 desert yellow/sand. I know that painting it all one color will look kind of plain, so I would like to paint it with several different shades to make it look somewhat worn. I have seen pictures of this kind of thing before, but I'm not exactly sure how to get that effect. I would guess that you paint the darker and then come back and shade with the lighter, or is it the other way? I'm sure some of you have seen this kind of thing where part of a panel looks worn and faded. I love this kind of weathering, but I'm just not sure how to make it look right. Can anyone give me any advice?