Hmm I didn't know about the X key. I've even been in inverted flat spins... cool stuff that.
I don't know if there is an standard method but here is what I do:
Chop throttle, nose down, opposite rudder, full flaps.
When the plane stops spinning and noses down, don't get on the throttle too early or you will induce a spin.
Like I said, there may be some official way to get out of a spin, flat or otherwise, but this works for me on the worst of them, even flat spins.