Normally when we are going to add crown we just don't edge at all and stop the paint about an inch from the top of the wall. However, I didn't know how long we would be living with the spaces without the crown and don't want to stare at that blank space for months. This provided a perfect time to practice. Low and behold, practice and a good brush is really what it takes. For me it was all about how to hold the brush. Here is a video that shows very accurately what the brush really looked like as I cut in.
I used a relatively expensive brush for this, a 2.5 inch Purdy Cub or "tight spaces" brush. Both have a very short handle.
How much practice? Our school room is about 13X18. I did two coats on the walls and two on the ceiling (so far, coat three on the ceiling is scheduled for today). I always paint ceilings first then the walls. My edging job on the ceiling was terrible the first time around. One visitor to our home actually laughed out loud at it. By the start of the fourth wall though I noticed I wasn't getting any on the walls anymore. So for me it seemed to take 45 linear feet of practice.
Now that I've also done the foyer (it was great to not have to tape off all of the doors and mouldings) I can say that it is much easier to cut in well on the walls than on the ceiling. In the foyer I was alternating ceiling and wall coats so it was important to me that the LAST coat be on the walls to cover any mistakes I made while edging the ceiling.
I'm not feeling cocky about it though. I still plan to hold off on the upstairs hallway which has seven doors to paint around until we are ready to do the moulding. That way I can paint after we've pulled off the old moulding but before we've installed the new. I have to go relatively slow when edging and it will be faster to not have to do all of that extra edging.