if you go down the video rout, its no good just making the one, showing it to the company, then not making anymore..... You need to persuade them that you will get their company/product known to other people.
Making lots of vids is a good way, it worked for Craig and Neil. But their vids are really well know for their riding, its no good trying to get sponsorship with crappy vids.
Another way is to enter comps and get some good results, companies are much more likley to sponsor a good rider who sometimes gets media coverage in mags due to comp results.
You also have to be really well behaved, no company would want to sponsor a dick, you need to act right, say the right things and so on.
To be honest though, unless your an amazing rider for your age, I wouldnt hold your breath about getting a decent sponsor, everyone wants it, but only a few get and deserve it.