Extensive bodyweight exercises and repetition of their art for hours at a time is basically the answer. As noted by Dave above, Christopher Somner's writings are generally considered to be the fundamental word on it. His website can be found here. It's prohibitively expensive to get an actual copy of his seminal work, Building the Gymnastic Body, although you'll find a lot of bootleg copies out there. Ignore the references he makes to future books in there. He scrapped his plans to write them and is now releasing his Foundation Series and associated works for particulars.
On a side note, one of the reasons he stated for scrapping the earlier books was that he realized that he was used to training people who were already strong enough to hold their own body weight at arm's length, and had underestimated how huge of a gap existed between the average person and where his book started.