What kind of tree are you making? Pointy evergreens are a cone scissor snipped to create the impressions of tips of branches; branchey trees like an apple are difficult with modeling chocolate alone because it tends to droop (and sometimes at the least desirable moments!) When doing this kind of tree, you may want to begin with some structure (edible like chocolate sticks or pretzels) or inedible (like toothpicks).
I like my sculptures to be edible, so when using modeling chocolate that needs structure, I paint it with chocolate after I create the shape with modeling chocolate, which helps it to keep it's form. I also like to play with a little cocoa powder accents to make a more interesting texture, appearance.