Practice is not necessary, you simply need to understand what is going on and be able to react with timing:

1. you are flying on control surfaces not wing and stab
2. you must be able to"catch" the airplane at any angle to keep it off the ground
3. we all learned to fly by watching the airplane then using inputs to control it, in 3d this doesn't work, you need to put stuff in before it is needed. for most people eye/hand coordination is too slow and sloppy to fly 3d safely and under control, but there is another way to accompligh this = a basic routine and timing. If you were on an assembly line with bottles flying by you could not possibly cap every bottle perfectly every time - you would fall behind and crash, so machines do this with a set routine and timing. this is the key to 3d
4. In my oppinion the rolling circle then slowing it down to a rolling harrier, is the first step, because you will become comfortable in "catching" the airplane at any angle
5. lets look at timing, anybody can learn this in one or two nights. take your favorite radio (mode 2 for example) and sit in a dark room or with your eyes closed (do not look at the radio) set the left stick at 1/2 throttle - begin moving left stick back and forth to end points passing through the center without stopping, at a medium speed get use to this maximum stick movement - dont hessitate, stop or release, just keep moving that rudder smoothly. now for the right stick - hold 1/2 left aileron constant and move the stick up and down to end points passing through center smoothly. now adjust the timing so that when the right stick passes through center on its way down your left stick is full deflected to the left. don't look at the sticks your brain will override the routine and you will crash, just keep moving those sticks - you are doing a nice rolling harrier, forget about what the airplane might be doing just burn this routine into your brain. I am not a computer just 42 years old with slow reflexes so I start all rolling harriers by turning on my routine then make small adjustments usually by holding on to something a little longer to make corrections or add a circle. but I never, never, never stop moving those sticks. if I become disoriented and I am 2 feet off the ground I keep my sticks moving in my routine untill I reestabligh visual control then release everything and fly out. It doesn't matter if I am hovering, torque rolling, doing knife edge stuff, when I am in trouble I always switch on my routine which slows down the airplane and keeps it from hitting the ground until I can regain control. In my travels I see many people do 1/2 or 3/4 rolling circles but loose it in the final leg - this was always my problem too until I learned this routine now when I get to that last part I swich off my brain or close my eyes and let my thumbs fly through it nicely - remember trust is a must. your thumbs will save you everytime, it is your brain that gets in the way. just my 2 cents, hope this helps, good flyin ray