How to take videos while I'm on a skateboard?

I'm going to New York on Saturday for the longboard bomb, and I'm bringing along my Nikon d3100. I've seen videos on Youtube of longboarders filming other longboarders while skating, and I was wondering the techniques they use to be able to look forward and not skate recklessly as well as be able to take good videos?

Usually use a helmet cam. A go pro with the helmet mount is pretty widely used. Fairly inexpensive, shock and waterproof. Small size. I wouldn't risk your Nikon while riding.

Get a buddy and first while preparing for the race find a good angle that he can put the camera against his chest and be able to skate without looking at the camera and is still filming you.

You can screw your camera to the end of a PVC pipe.
You can also use a GoPro and screw it to your helmet.