The first thing to consider when choosing two trees to tie the tarp to is to look up, to make sure no branches can fall and impale you while you're sleeping. Then you look at the ground to see if it's flat and there are no game trails or ants nests where your head is going to be :). In our country, our weather systems and wind come from a south westerly direction. So your tarp wants to be at least within a 45 degree angle with the back facing toward the wind. Erect the tarp at a nice angle and the wind should flow right over it and the rain slide down it.
I think that if you have a tarp then it can be used as a quick fix resting spot in the right conditions. I will be using, investigating and making alternative types of shelter and writing on blog about the processes used for each one.
Here's one I made earlier.. :)