Something you could add is specularity for the sun.
It is very easy to add but improves the look a lot wink You can check out a tutorialchapter about it here: http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series4/Specular_highlights.php

Lite-C Tut. 1 Lite-C Tut. 2 Lite-C Tut. 3 (en)
Shader Tutorial (ger)

http://www.slindev.com