Sorry - managed to miss this thread first time around!
I haven't done any mountain biking photography for a few years, however got a good few tips 6 years ago from a magazine article written by Steve Behr, who at the time did almost all the photography for MBUK.
Almost always use flash, what you will find is that with the peaked helmets people tend to wear, it casts shadows across the face, so get down low and shoot up, with a blast of fill in flash to lighten the shadows in the face and enable you to capture the facial expressions. My pet hate in my pictures used to be a shot where I couldn't see the eyes!
By getting down low you will also make the rider appear larger than life and give them a real presence in the shots too.
I don't know about settings on your camera etc, however also experiment with 1st and 2nd shutter curtain sync for the flash. Combined with a slow shutter speed and without the lens wide open, you will find that this produces a nice blur effect from the slow shutter speed and then the burst of flash then freezes the motion. The smaller aperture value also will reduce the ambient light captured so it will show the background as darker, increasing the pop of the rider in the picture.
I tended to find that with a rider coming towards you, freezing the action with high shutter speed was the best approach. With the rider going past you then will find that panning with the rider and using a low shutter speed works well....it also means you can be less selective about the background that you choose too.
Main things though, get down low and use flash to fill in the shadows (if you have flash exposure compensation, dial in about -1/3 to -2/3 of a stop.
Given you are shooting friends then experiment and find what works for you best. Take the shot, chimp it and then get them to re-run the same trail again so you can perfect the shot you want.
The lens you have will work well, try the wide angle settings too, when low down and shooting up they give rather a good effect.
Finally, have a look for posts in here by a chap called Transcend (I think that is the name) - he is rather good at the whole MTB photography malarkey!
Good luck and make sure you let us see the results!