The Wargames Table has a nice article on modeling trees for The Great War. The trees also look to me as though they would be nice for a deadlands type of fantasy setup.
Mark Veneman has a tutorial on making model trees. He also has a useful selection of tree silhouettes to help you shape your little wonders. While the instructions are aimed at model railroaders, they’re just as useful for miniature wargamers.
I am in the process of making some new tree stands, so I went looking for some advice. I like the way these turned out.
Dramatic Katastases has a nice, if brief, post on making stands of trees. I think I’m going to convert a few of the trees I have on washers over to this setup. They will be more stable and add a little something to the table.