Ok, hope I can give you a few tips here. This is basically what I've learnt from other members since joing epz over a year ago.
Shoot from a low angle whenever you can, whether you intend to silhouette the subject or illuminate with flash.
Try to ensure that both the skater and board are both isolated against the sky.
Although timing is essential and it looks like you have no problem with that, you still have to bear in mind the background. Try to make sure you keep horizons level. If you can shoot at a position where there's no one else in the background (not always possible I know) then this can help a little.
Hope none of that sounds insulting, just trying to pass on tips.
Check out my mate Seb's
work or Frank C
for some quality skate stuff.