I find flat spin entry is easier if you start with a normal spin, then increase power while slowly moving the ailerons to opposite deflection than for a normal spin. For example, for inverted left spin, full left aileron, full right rudder from inverted. Get spin started, then go to full right aileron. I have never had any problem with recovery just by returning controls to center position, but have heard some pretty bad horror stories. Flat spins tend to take more than usual elevator deflection - using dual rates help a lot, although I don't like using dual rates all that much, but they are very helpful for the more extreme stuff, such as so-called 3D aerobatics.