Ok, posting this in here at the request of Ravells, although I have to admit I feel a little weird doing so, it hardly constitutes a tutorial, especially amongst all the other great threads in here.
Anyway, that said, here's my little tip for creating a quick, hand-drawn style forest with a kind of, hopefully, seamless manual tiling. (This is copied over from this thread: http://www.cartographersguild.com/showthread.php?t=5397)
When I was looking into using a multi-tree brush on my map, I found one good way to do it was to stagger the edges of the brush. Basically, you make your new image, fill it with your tree brushes but at the edges alternate between a tree and a tree-sized gap. That way, when you lay out your forests you just lay the pieces like a jigsaw. It's obviously not as fast as filling a selection with a seamless pattern but it does allow for a little more variation to fool the eye by placing the pieces staggered and maybe leaving some little clearings. Then just go around the edge of the forest and place some individual trees to mask the fact it is done in squares and voila, a nice forest.
No need for feathering to try and mask that it is a pattern and chop trees in half in the process.
Start by making some quick tree brushes.
I made about half a dozen like the following...
Then I whipped up a quick brush from the smaller brushes smaller brushes. As you will see I staggered the edges and added a little pencil-like shading in between which looks very rough but as I am making these a lot bigger than I need it looks ok when they are made smaller.
I then sketched out a very, very quick forest with this brush. It took me about 10 seconds or so. The brush isn't perfect so there is overlapping in places but that can be fixed with more care spent over the original brush and with better placement. I also added some single trees around the edge with one of the brushes I used to make the main forest brush.
As you can see, it can look very effective with minimal effort. At least I think so. The staggered edge helps mask the fact that this was made using a shortcut technique. Certainly you have more control than over a seamless pattern fill.
I hope this helps someone.