Sharp, Cottonwood and Cedar are the two best woods I've found, I've actually used more Cedar myself because we are surrounded with Cedar trees. We have a local state park called Cedars of Lebanon. <img src="/ubbthreads/images/graemlins/wink.gif" alt="" /> Ours are Red Cedar though. I've experimented with several other woods, some like Pine are too soft, and others like, Peach, Oak and Maple are too hard.
Yes I have made some cordage from a few different fibers, like Thistle, Oak bark, Rafia, Milkweed, Cedar bark, and I've tried a few various grasses, vines and other innerbarks. It takes as much or more time finding the right materials as it does learning to make the cordage, but you learn a lot along the way. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
I have to admit that I use leather or nylon strings on my bows most of the time. Most plant cordage just can't take the stress of the firemaking process.
It really does take a lot of time and dedication and stamina to get the coal going good, so don't give up to easily and make sure and take long level strokes on the bow.