Just take time. Like you said, first day out with your camera. Ironically enough, some of those were the first time out with my DSLR too. haha.
Some tips I have learned or know of:
1) Run your lens on AV mode. Use an aperature speed of 2.8 or 3.5. You want your shutter as fast as possible to freeze the action.
2) Mid day sun isn't the best. If you can get out early morning or late afternoon, the angle of the sun adds for more contrast and shadows. Use this as back light, or to illiminate dark spots.
3) Positioning. You need to be in the right spot on the track to get the angles you want. In that black and white picture I was at the apex of the turn in the trees. Some riders were actually hitting me as they went by. I shot those at 18 mm because that was the widest angle I had (my kit lens)
4) Light post processing can be many pictures justice they deserve. Play with the levels. Increase saturation. You'll find those two things alone will make a huge difference. A good example is rider 716. That simply was increased saturation, some increase in the lower levels to add contrast and darken the darks, and then the lights were brought way down to white out the background. There actually was a faint view of the finish line table in the background. The increase (lowering) of the lights made that dissapear and added an interesting view of the picture. It somewhat washed out the the horizon and rider, but still a neat effect done in about 10 seconds.
5) I don't get permission from riders, but have yet to have anybody upset. If a pro is out testing I refrain from taking pictures. I'd like to, but I don't want to be hassled. Either way, with this in mind, nobody's come up to me with any problems and some actually put on a pretty good show when they see people taking pictures of them. For example, the picture after "Some with post processing" comment, the guy always went huge when he saw me at the corner. Great for pictures. I didn't get a chance to talk with him, but thought it was great for the show he put on.
6) Lastly, have fun! I go out and take pics of my buddies when we go, often when I don't have time to prep my bike or want to take pics instead. We are more trail orientated so most of their pics don't get selected for further consideration, but there are a few. The black and white image is one of my buddies on a EXE. He took a hard spill the last day we were out and will take a year or so to recover. The picture was taken with an on camera flash in the corner with dimming light.
I hope to get out this year with two or three flashes and getting permission from certain riders to take pictures of them. Hopefully come up with some professional looking pictures.