After thinking about it more, I conclude that the only thing that "inspires" me to work is to have something on the canvas. As someone just said here, work is its inspiration.
I can relate to this.
Mostly, I get a few big mistakes on the paper/canvas/wall/screen and have plenty of inspiration to creat what I had originally intended.
My advice to ogviok (sp?) is to make mistakes, and be fearless. You must first be a master of the medium, and then after that you can form a bond with it and try and communicate. If that makes sense.
Of course certain technical abilities will enable you to enjoy your drawings more.
For trees; I would recommend learning about what they're composed of, how they actually take the appearance they have. And then try to build it out of macaroni or craft sticks or something.
Make it your goal to draw a blueprint from multiple angles. After you did th print and craft stick sculpture you should have an easier time with the tree drawing.