Yeah, I've actually started to use a couple action figures that I have that have some moving parts and this is good for some things.
One thing I notice about where my struggle is, it's in the way the appendages and 'outards' of the body move as the action is being executed.
So it's easy enough to get a reference and stills of a side view of a flying kick, but it's much harder to get the 'windup' action poses and frames correctly. So if a fighter is running into a jump kick, those moments right before, as, and during the air before the full extensions of the feet seem to be the hardest for me to accurately portray, and also seem to be the hardest to find references for.
This is why I really like the site Kasumi posted, since it shows the full animation of idle -> windup -> action, something that would be pretty difficult to get right on a mannequin. Only problem I have with that site is the videos are flash and I have to convert them an rip the frames which is a bit tedious. And they have limited animations, so clever, tricky, 'super' and gimmicky fighting moves are not featured.