I've been doing pretty well on the sim using the method that FlyingCirkus.com suggests where you are giving rudder and elevator inputs simultaneously. They say to do this and first learn the rolling harrier circle. To straighten it out and make it look smooth axially, I've found that practice makes the timing of the rudder vs elevator get smoother. Also, throttle variation helps to smooth it out and keep the same angle of attack. I'm not sure the Flying Cirkus method is the best for a perfect harrier roll but I just couldnt keep up fast enough doing each input individually....