I have been practicing them quite a bit over the last 3 months and am getting low and slow with them.

The trick I have found is you dont want too much elevator throw or you will be too high of alpha or in some models you will snap the wing. You also need to start with rudder then add the elevator. Dont bang the sticks at the same time, this will make them very jerky and not fluid.
Also throttle control is critical and is what will give you a higher alpha. I hit the gas a bit on the inverted sides too keep the nose up.

As BarryB stated ,try it in a wind it is actually easier.

I was just doing them tonight before sundown in a 20mph and it is fun holding the plane in one spot while rolling.
P.S. Be caerful on the downwind side,you loose half your authority as you slide with the wind. It almost got my buddy tonight.