Drawing leaves can be a challenge. However it is possible to ease the trouble a bit. I have put up little tutorial “How to draw leaves” which explains how to draw simple leaves in various positions.
I did two short videos as well to illustrate how simple leaves can be drawn by using the very basics of plant anatomy.
Recently a fellow artist (Albrecht Rissler) posted an interesting link that fits in here quite well. It is a blog post : Mother nature decoded by James MCCullen, who blogs on NYT (line by line) . McCullen gives a good example how the leaves of a tree make up for a lot of the look of whole tree.