All you really need to do is get the CG back really far and increase the control rates. That said, some will still do it better than others.

There was a trainer at my field doing some of the best looking flat spins I've ever seen. It was pretty tail heavy, and had 4lbs of lead strapped to the bottom of the fuse. (yes, you read it right, 4 pounds of lead. Why? Because he could, it was fun, and a challenge).

Anyway, the plane spun very very well. It spun so well, it didn't really like to stop spinning.

In fact, I saw him twice flat spin it right down to the ground, in the middle of the runway. I heard he did it a few other times, when I wasn't there personally. The first time, the damage was very minor, bent the gear, and the lead all fell off, but that was it, the spin was that flat. Eventually the plane was destroyed. (Which was part of the fun of it, after all).