You can also pull your tarp in closer and/or pinch the ends in if it's big enough. This doesn't give you a ton of additional warm, but it helps. I use a Kelty Noah 9x9, so I have plenty of length if I pitch it on the rectangular to pinch the whole thing in to help cut down on wind and preserve some sense of ambient temperature.

I never thought about hanging the tarp low and piling up leaves. That is a real good survival idea should the temperature drop significantly lower than you planned for.
Why am I online instead of hiking?