Nylon-based tarps tend to stretch during the night and when they get wet. This can be a potential problem for small asymmetric tarps because an edge sag could direct water onto your hammock. The solution is to use a tarp line tensioner—an elastic cord (e.g., shock cord or elastic medical tubing) tied to the tarp and to the guy line—to take out the slack during the night.
A simple, yet effective (and inexpensive) tarp line tensioner is the humble rubber band. You can hook several small rubber bands together or just use one large rubber band. To install, push one end of the rubber band through the tarp tie-out and then feed the rubber band through itself to create a Lark’s Head knot. Take the guy line and tie a sheet bend on the standing end of the rubber band. For added security, leave yourself enough guy line to tie two half-hitches or a bowline knot on the tarp tie-out with enough slack so the rubber band will stretch but not break.
NOTE: Rubber bands tend to crack and get brittle after long exposure to the sun. I wouldn’t consider rubber bands as a long-term solution, but they are certainly a quick and easy way to try tarp line tensioners. I have more information on tarp line tensioners on page 80 in my book, or check out HammockForums.net for detailed instructions on making robust tarp line tensioners.