The best way to create tree (or any other kinds of) leaves is with a Vray2SidedMtl. The reason is that leaves are very thin and let through a lot of light. This is especially visible when there is any sort of a back light.
Basically, you’ll need to set up two materials, front and back. The front is a strong, saturated green, with slightly glossy reflections (increase the glossiness for a wet look), while the back is usually lighter and more desaturated, with less reflections/glossiness.
Setup for Front mat:
Back material:
Use a normal map for Bump. Front material has a positive Bump value and back material has a negative value. This gives us a realistic shape for the veins and the rest of the details.
Combine the two materials in a Vray2SidedMtl. Use a translucency map like the one below to make sure the veins are not letting through as much light as the thinner parts of the leaf.
Here is a full view with a backlit render to show the material in action:
