I have found that if you have the ailerons going down when the elevator goes up, this makes harriers and elevators almost
impossible to do. I think that the ailerons going downcreates more vortices which increases the lift of the wing and reduces he stall speed, but make the transition bettween low and high alpha flying a lot more tricky. I found it a lot easier when i had them turned off. If your wings start rocking when you are the right way up, try doing it upsideown- many planes do not wing rock inverted.
Another thing that also seems to work is to increase your rudder elevator movement to get the plane in a deeper stall.
If this is not quite enough then try entering your harrier from a snaproll on full 3D rates. This often gets the plane very deeply stalled without wing rock,without the need for any mixers , and is also a bit more cool looking. It is also easier, because the deep snap roll transitions you seemlessly into a harrier or an elevator, once you have got your setup right and have practised it. You can see this manuver done to near perfection in one of Jason Shulman's videos on www.scaleaerobatics.com
Luke