Cool.
If you need to pull a lot of rope, you can also put a prussik on the rope near the pulley attached to the tree (provided it's a prussik-minding pulley), and attach said prussik to the same tree. See image there:
http://en.m.wikipedia.org/wiki/Z-dragAfter a pull, this prussik will act as a ratchet, allowing to move the first prussik up the rope towards the load, in order to pull some more. Note that this second prussik will hold the entire load when moving the first prussik, so care must be taken to act smoothly so that it doesn't snap.
One key word in the article that is not apparent on the photo is that the most effective way of pulling is with all lines parallel, unless you add more pulleys. That's why the minimum number of pulleys for a good Z-drag is two, but a third one might be useful if there isn't enough room to pull in line.
Of course such a mechanical advantage system can be set up without pulleys, only with carabiners, but then friction will negate some of the mechanical advantage.
--C.
