The easiest thing to do is to repaint them with one of the new "for plastics" spray paints. I have used FUSION and I noticed that Lowe's/Home Depot are now carrying another brand that comes in tons of different colors.

I'd just scrub off the dirt and repaint to cover the stains. The Fusion that I used on some chairs a couple of years ago has held up well.

Try Mr. Clean Magic Eraser found in the cleaner section of your grocery store. It is amazing what these little sponges can remove!

Rebecca

Another vote for the Magic Eraser. Those things are wonderful!