Trees are not cardboard cutouts with brown trunks and leaves that are simply green if it's summer, red if it's autumn, or absent if it's winter. The 'secret' to painting believable trees is an understanding of the underlying structure of trees complemented by observation of different species.
Compile a file or sketchbook with your notes, sketches, and even bits of bark and leaves. Buy yourself a tree identification guide (a comprehensive one, not a pocket one) and learn the names and characteristics of individual species. Read the descriptions in the tree guide and compare it to what you're seeing.
Another place to learn more about trees and tree identification is the Foresty section on About.com, starting with the articles on Basic Tree Anatomy and Identification and how to begin a Tree Leaf Collection. If you're living in North America, you should also check out the Forestry Guide's Recommended North American Tree Identification Books.