It may be a little presumptuous of me to be posting a tutorial with only a handful of posts to my name, but I after struggling to make a decent looking forest with my neophyte photoshop skills, and finally succeeding in making something that didn't look like a horrendous mess, I thought I'd share.
The whole process starts with a very particular brush, namely the 'Rough Round Bristle' brush. Once that's selected, we can begin.
Step 1: Open a new layer, and using a quite dark green, paint on where you want your forest.
Step 2: Right click on the new layer, and choose 'Blending Options' and then click on 'Stroke'. By default, this will add a bright red border around your forest, which is no use to man nor beast. Click on the colour box and you'll be taken to a colour picking screen. Choose one of the greens from the bar of colour in the centre, and then pick a slightly darker green than you used for your forest. Depending on the size of your forest, you might need to change the size, but for this example, we're leaving everything as it is.
Step 3: You should now have something that looks like the third image.
Step 4 (Optional): This step is entirely optional. It adds a bit of texture to your forest, which may or may not be appropriate for the look of your map. I'm including it for completeness sake.
Right click on your forest layer again and head back to 'Blending Options'. Click on 'Pattern Overlay' and a weird bubble-like texture should appear all over your forest. Don't panic! This is quite normal and is easily changed. Next to the little preview window for the pattern is an arrow, click on that and you can choose a different style. I've chosen the 'Wrinkles' pattern, which is right next to the default bubble one. Once you've selected it, change the opacity of the pattern down to about 10%, and the scale to about 50%.
Step 5: Still in the 'Blending Options' menu, head to 'Drop Shadow'. The default options are largely fine, but you might want to increase the distance a little. In the example, I've increased it to 11 pixels.
Step 6: That's it! If you've followed the steps, it should look something like the sixth screenshot.
The advantage of this method, especially for novices like myself is that, thanks to the 'Stroke' we used, we can simply erase sections of the forest to make clearings or make a path for roads or rivers. Just use an irregularly shaped brush, like one of the chalk brushes, to maintain the 'foresty' edge to the stroke.
One world of warning though, you may need to reduce the distance of the drop shadow, as it can sometimes stick out a little too much. See image seven for an example of both the erasing effect, and the 'sticking out' shadow effect.
You can also easily substitute other colours for the greens I've used. For example, If you're going for the 'olde worlde' look, just use browns or tans.
Anyway, I hope you find this helpful!