If you're in a wooded area that allows it, I would suggest making an a-frame out of your tarp. This seems to help keep the wind and rain out a bit better, and you don't have to worry about water puddling in the middle.
1. Tie a center line between two trees about 7 foot high, and pull it very taught. I'd suggest a truckers hitch on one end, the other end doesn't really matter.
2. Drape the tarp over the line and tie the corners down to the ground or to some lower trees.
(optional)
3. If it's raining you may have water dripping down the center line, you can tie a couple small pieces of cord around either side of the line just under the tarp. This will make the water drip off at the cord instead of in the middle.
4. If the tarp bunches up in the middle of the line, you can tie the middle of the tarp off to the same trees the center line is attached too. Using a separate line for this will help keep tension off the tarp so it doesn't rip as easily.
(pulls out the ascii art, #4 not shown)
\______/ .-Drip cords-. \______/
|| \_____________\ ||
.--oo-----/| /|----O----.
'||'| / ' / '\ |-||'
|| / / \ ||
|| / /------o ||
|| o------------o \| ||
|| |/ \| ||
P.S. Please be kind to trees and don't destroy the bark. If you're pulling anything very tight put some sticks between the rope and the tree.