here's how i do it and it usually works.
most aerobatic aircraft will want to rotate with the torque of the engine, so use that to yoru advantage. one thing you can do is pull a sharp down or up and give full rudder while holding your up/down and it will usually enter the spin. see, flat spins are easier to enter when your aircraft is stalled, and wont end up in a steep spin rather. so while your wing is stalled completely, your rudder is not so while you are stalled and your airspeed is too slow for your tailfeathers to make a big difference as far as keeping the plane true goes and your power is high enough to vector it, you have the perfect setup for a spin. try it.