Here are a couple more pictures and text to help you make your model more realistic looking (courtesy, "Hobby Japan Mooks"):
If you are going to "dirty" the model using pastels, take some sand paper and powder the pastel color you want to use. Use a dry brush and gather up the pastel powder as if you're scooping it up. Use a brush with a wider, longer tip. It's easier to pick up the powder that way.
Then, with the same type of touch as dry brushing, touch the brush up against the corners and edges. Once you think there's enough color, lightly blow off the excess. (Be careful of over blowing. Spit will take that powder right off.)
As the name implies, we are going to use a dry brush to place color upon the plastic. But not as the name states, you do not want to have a completely dry brush. Ideally, dry brush is best used when the pain on the brush is *right* at that point of being completely dry. We are using silver in the picture.
Now after we remove the excess paint from the brush with the tissue, we want to target the edged regions of the model. Think of it as a sawing moving with the brush. If you get too much paint on the brush, then wipe it off and try again. Try not to leave brush strokes.
The third picture shows the final product. Normally dry brushing brings out the bumps and indents of the model, but this time we've tried for a "scraped up" look. Remember not to over do it though. It won't look real then.
By nimbly touching the edges of parts with silver, we can show an effect called "balding." That is simulating the effects of three too many beams to the shield or just clumsiness (oops, sorry about that GP-02, didn't see you there).
You don't just have to use enamels to weather. Below are an alternative to brush work:
The above image shows us performing the oil drip using a thin detailing Gundam Marker.
Finally, here is the aforementioned balding effect with a thicker Gundam Marker
