Toning? Depth? HELP!!

[Dr. Phils apprentice]

Hey everyone!

Im pretty frustrated not knowing how to "tone" your drawings, can any of you guys give a couple of hints??
http://i147.photobucket.com/albums/r283/jensen-the-man/Billede001.jpg

Any help would be appreciated! :teeth:

[Dr. Phils apprentice]

PS; sorry for the poor quality :(
Sucky camera!

[Seedling]

I just added some value exercises to Concept Art 101, if you are interested. The link is in my sig. . .