I just recently got back from a trip where the only two trees available were 18 feet apart. I was using the straps I'd gotten with my hammock,(ENO Slapstrap PRO) which according to their website are made of nylon.

In the morning I found myself 3 inches from the ground!! Before I went and crawled inside the hammock it was at chest height and I'm 6'4"!!

I did some research and found my way to ED Speer's website and bought a 30 foot roll of 1" polyester webbing and haven't had a problem since!

Kudos to Ed, for a great book, I had referenced it when I got back and he recommends polyester for that very reason.