For one thing, your sensor wants some cleaning. You can see spots in the sky on
IMG_0950 on Flickr - Photo Sharing!
The photos were taken at 55mm and 18mm, my bet is that you're using the 18-55mm kit lens. All zoom lenses lose quality at their extreme zoom setting, so try to avoid that. The 55mm photo was taken at f/5.6, the widest aperture at that zoom, which also won't give you the sharpest results. Experiment a little with smaller apertures and stay within roughly 20-50mm, and I'm fairly confident the results will be sharper.