From the manufacturer's website: http://hennessyhammock.com/articles/tips_from_users/
Ideas to keep your rain fly tensioned
You’ve probably noticed that nylon tarps, tent fabrics and nylon adjustment cords will stretch when wet which often results in a loose and flapping rainfly, which later tightens up and looks great again after the sun dries it out. Nylon stretchess 11-15% and polyester stretches 2-3%.You can tie a few loops of shock cord between the “O” ring and the nylon adjustment cord on each side of the rainfly. Use the stretch in the shock cord to take up the stretch in the nylon when it gets wet.
Improving fly shape
Always tighten the rainfly lengthwise first , then tie the side adjustment cords out or down last. This will resulty in a better shaped rainfly. If you want to tweak the shape even further, you can tie a length of lightweight nylon string lengthwise under the rainfly between the two "O" rings. Adjust to tension fairly tight and it will hold the fly a little higher, take out a few wrinkles and improve air circulation under the fly.