In addition to the above, one may wish to also consider training in yoga and or any other physical activity that promotes endurance and flexability.
A lot of people seem to be under the impression that a submisive or slave must be absolutely motionless when kneeling, but that is simply not the case. Sometimes the subtle movments (little stretches, shifts in position, isometric excesises etc) a sub or slave makes while kneeling to maintain circulation or increase his or her's state of readiness is considered to be more apealing than the actual posture itself.
(I only flip my toes back and sit down low when I know I am not going to be summoned from my position for awhile, at other times I am pearched on the balls of my feet ready to leap up)
When I was in training, for those of us unable to kneel well, they had two forms of practice available in addition to regular positional instruction and physical conditioning. One was the introducton of a rolling pin like piece of wood (sometimes padded sometimes not) that one could tuck between ones bottom and where ever most comfortable accross the calves, (some girls were even permitted small pillows) which elevated your angle and made kneeling for longer periods more tolerable. The other was a small stand that one could be tied into where ones legs were stretched up under oneself and spread at the knees but seperated by the "seat" with the lower legs kept from touching the ground so all your wieght was on the botom of the thighs. (I hope I explained that thing well enough lol).
Whatever you do, dont shoot for the moon right off, take things slow and be safe, not everyone can be an accomplished kneeler, especially those of us who start bdsm late in life or dont come from cultures where kneeling is very common.