Ok, after seeing Arachne's and st0ven's trees I got inspired to do a little step-by-step myself. The tree I ended up with isn't as good looking as theirs, but I've taken a simplistic approach I think should be pretty easy to follow and hopefully helpful in understanding how to draw these things

e4r: The feeling I get from looking at your trees is that you're putting too much effort into shading the foilage and not giving enough consideration to the structure underneath. But as with so many other things, to draw trees right you need to figure out how they "work". If you study Arachne's tree you'll see her leaves are really just splotches of pixels: by themselves they don't look like anything. It's how they're put together and shaded
as a group that not only gives the impression of looking at leaves, but also suggests the form underneath. St0ven's tree is done in a different style but the same applies.
So how do trees work then? Basically you can think of a tree as one big branch growing out of the earth. This huge branch (the trunk) grows upwards and gets thinner as it grows. As it gets thinner it will err, branch off into smaller branches which again branches off into even tinier branches until you have the twigs. At the tip of each tiny twig a leaf grows out. Fair enough. So let's draw this huge branch then:

The beauty part here is that you don't really have to worry about drawing it wrong. A tree's growth is random so you can draw it randomly, just make sure you draw quite a few branches and twigs. And one thing that's handy to keep in mind to keep the tree looking natural is to not draw branches side by side like this:

But rather like this:

EDIT: Oops, hit post when I was supposed to hit preview

Be back soon with the rest

Ok so we have the trunk sorted out. It's time to draw in the leaf clusters that will make up the foilage. Let's create a new layer and add in some splashes over the twigs:

Notice where I place those splashes? They're placed to encapsule all the twigs at the end of the branches as that's where all the leaves grow from. Here's another image with blue circles for clarity:

No need to be too concerned with the shapes yet, drawing them in to cover those twigs at the end of each branch is the important thing right now.
Next I've created a new layer and drew in lots and lots of small blotches, all using a single colour a bit brighter than the splashes. Again, I didn't put too much effort into getting the shape right, I just made them roughly leaf shaped. I also took care to have them stick outside the splashes as seen in image 2 below.


Created another layer and drew in more leaves in a lighter colour: This time taking care to put most of them at the end of each twig like so:

The reason for doing that was.. well, I guess I did it to make the foilage seem more like its actually growing from the tree and not tacked on afterwards

Yeah we'll go with that
And here's the tree we get so far with all layers visible:

It's shaping up but still looks kinda flat. We need some highlights to give it more volume. I added another layer on top of the others and drew in some more leaves with a lighter colour than the last:

Note that I used them sparingly and mostly at the top of the clusters (indicated with those blue circles above) because the light comes from above so that's where it will hit.

And there it is. Over-donewith-gone

Phew.. longest post I've ever written! It isn't perfect, still lacks some dimension I think because there are no branches pointing towards or away from the viewer, only left and right. But what the heck.. It'll do. Hope that helps a bit

EDIT:
Added another step to this. I concentrated the dark and midtoned leaves around the clusters/light-toned leaves some more to make the transition from dark to light a bit more seamless and pleasant to look at. In image no 2 the old tree is at the left for comparison.

