try this, get a good quality angle latex Purdy or Baker 2 inch brush, sometimes called a sash brush.
working from left to right, with the brush dipped in paint, touch the area and then drag the brush up to the intersection and slowly drag it on a straight line for about 6 inches, then stop and re load paint, then go another six inches.
You are right the blue painters tape is not real good for this job, because one leaves gaps on the tape and paint will seep under and distort your line.
What you can also do is to add molding at the intersection which is pre painted before install and this will make the room look great and you will not have to worry about that straight line. I am either speaking of crown, or if crown is out because of the special cuts, then just floor molding nailed to the wall area butted up to the ceiling.
Your question and job does require a steady hand, experience and a good quality brush. A cheap brush will never do this task.