Another thing I found difficult in the beginning, and which I notice a lot of other tree-beginners find difficult, too, is ending the tree. Roots may go fine, and the trunk, but the top ends up looking like the tree's been cut from halfway up and there's branches growing from the stump.

Branches don't grow just from the top of the tree*, and they don't all necessarily point up. The cut-look is fixed by remembering that the tree gets narrower on the way up; it grows by creating new layers on itself (yes, the growing takes place from the inside, pushing the older layers into a wider and wider cylinder around the new ones, but that sounded simpler somehow), and in the process the base gets thicker, and the top higher.

*of course different kind of trees have different kind of branches, though. I mostly draw deciduous trees, but for example spruces and pines differ a lot in how high you have to go for living branches.