Shakey, this looks like a good start. Yes the dirt is a pain in the bum i can tell you and i have many a time taken a picture and then put camera under my vest till the dust settles but always still gets lots of dust.
The pic is fine by looking at it not too much shutter speed so the wheels have motion blur but this is where you need to try and get a better spot as sometimes it's all to easy to shoot into the sun and causes a shadow on 1 side there fore you need to use a flash for fill. Sound silly, but it isn't we cant always be in that perfect spot on some tracks so if you happen to be on the wrong side try using flash. When i did the state rally here i didnt have my SB800 flash and well if i had it i would have used it as i was stuck in a pine break and was getting rather dark but you will soon learn the more you do and try different things.
here is a couple i took then.
Anyhow practice, practice and more practice.
Cheers
you are off to a good start