There's about 5 different ways to do this, and each plane only seems to be able to do it one way. The way mentioned above works for some planes. Others do it better when doing a hammerhead. Once you rotate half way around and the nose is horizontal, slam full opposite rudder, down elevator, and a little oppose aileron. It'll settle in and just fall.

Another ways is from the vertical, quickly slame full down and make the plane nose down. As soon as the nose is pointed towards the ground, but before it gains any downward speed, give it full rudder, full down, and a little opposite aileron.

Just takes a little experimentation to find out what works.