boomsjakie wrote some time ago that fx runs a lot, that's what makes his stamina able to do combos over 100 tricks. It's a good way I think, but not the only. Imo if you focus on doing long combos only you can get them longer and longer just by training them a lot.
Adding a lot of jumping tricks, like tatw, amatw, aatw into long combos may increase your stamina as well