get closer to the subject... not shoot into the sun, get an off camera flash or 2 i think its in the beyond section of this forum, i have a thread called "calling all skateboard photographers" thers some good advice in there
Good work. I'd suggest using a shallower DoF to help isolate the skater from the background and shooting either later in the day or earlier in the morning for better light. Don't forget to crop your images so that the subject isn't always centred.