Gidde, myself, and a few others have some tutorials on making mountains. There are quite a few others also for mountains and trees. Also, the in the resource section, there are quite few premade tree brushes as well as mountain brushes. of course, if you want a hand drawn look... you have to hand draw it.....

Also, a lot depends on your image manipulation tool of choice (or necessity if you have a price limit and no software already).