Be flexible. It came to my awareness the difference in the capacity of my skills relative to those of my cohorts. the difference is in the ability to be flexible.
During my prime, I used to stretch a lot, before class. I did my own usual warm ups. and I stretched and stretched myself. every single muscle I can find to stretch, I stretch them. Then I joined some other martial arts and learned other forms of stretching, and incorporate them. In doing so I also learned new ways and range of movements the body and joints have, and properly prime the body to undertake a high stress situation.
That was my foundation, laid by myself in the past, which prepared the present me to do what i can do. There is no magic. Its all and investment paying its dividend now, although I stretch less than what I did when i was younger, and I am a lot stiffer now, some how the memory of that flexibility remains, and no matter how Harry sensei throws me, i will somehow find a way to land safely, or minimize injury.
The other form of conditioning that I feel is essential is core strength. the trunk of the body, connecting the legs and the upper torso. I felt that when I am taut in the mid section, I am able to project strength consistently. It also allow me more control over my ability to flex. Call it being centred or concentrating on my one point, physically if i am tight in the abs, i can will my body to enhance its capability.