Nikon D7000 lens for filming plus tips and tricks?

I'm after a good lens for my Nikon D7000 for filming things like extreme sports. BMX, skate, 4wd, motocross etc.
I'm after something that won't constantly be hunting in and out of focus as they go past or as I move around them as I will be using a glide cam for some good shots.
Any tips, tricks or pointers would be handy also.

First you can't do "filming" with this camera because it doesn't use film… It shoots / records video… No film.

I don't know which lens you're using but auto focus is only so fast and can't adjust to moving objects.
My suggestion would be to try manual focus, grab your focal point at about mid range where your bikes will be and shoot on burst. You might get lucky and catch the in focus.

Also, if you have enough light shoot at at least 1/500 if not 1/1000 for any stills you want. For video you're going to have to take your chances.

Focus hunting is more of an issue with technique than lenses.

Focus hunting occurs when there's too much variance between the subject you are wanting to shoot and the background. An example is photographing an airplane in the sky. The camera does not know you want to focus on the small object, but thinks you may want the sky in focus.

This is solved by using spot focusing on your camera (the default is matrix focusing).In spot focusing, you can select an individual focus point and that is what the camera will focus on.

I'd suggest using both AF-C (Autofocus - continuous) and spot focusing for sports.

The D7000 has nothing to do with film.