Ed_Moorman's point 3 is the flat spin recovery technique that should do the trick.

Not all aircraft will come out of a flat spin directly. Try going back to normal spin controls (in the direction of the flat spin, e.g. for an upright flatspin to the left that will be full up elevator, full left rudder and in some cases full left aileron) and then wait until the flat spin becomes a normal spin (it may also require a little bit of juggling with the throttle, but in most cases I have simply returned the throttle to idle). The hairy part is to wait for the flat spin to revert into a normal spin, it may take several turns to do so. After the aircraft is spinning normally one can use standard spin recovery techniques.

/Red B.