mostly comes with experience, and knowing the diameter of your thumb. for me it was a lot like bows, just exploring the possibilities and figuring out what i liked and didnt like gave me a much better picture of what i liked than what people told me to try. my personal favorite when it comes to thumb rings is a buffalo bone one given to me by my soon-to-be father in law, I dont use it very often, on account of it's significance to me, but a close analog is my cattle bone one. though it will eventually wear out, i also use a horn one from time to time, as i find it has a fantastic release, but as it was earlier stated horn will wear out over time. it makes me wonder how something like lucite would hold up as a thumb ring, i'd imagine it would perform quite well.
