Many years ago when I was painting my ceilings in my mobile home for the very first time, I asked the saleswoman at Sherwin-Williams about how to paint along edges without slopping paint everywhere or spending huge amounts of money on masking tape. The saleswoman recommended that I buy a 10" Drywall Taping Knife to use as a paint edger. This is the best painting tool investment I have ever made outside of the paint roller tool.
I still use blue painter's tape occasionally when I can't angle the Drywall Taping Knife adequately to get the trim brush into tight corners. But otherwise, the Drywall Taping Knife is very quick and convenient to use. I just jam it into the corner and angle it about 60 degrees, then brush right along the edge of it. This technique keeps my ceilings white and my walls bronzed, and because it's not sticky tape, I don't have to worry about fresh, uncured paint pulling away from the ceiling when I pull the edger away. I don't have to pay $4 a roll for painter's tape that I can only use once either. I keep a wet paper towel nearby and wipe the paint off of the Drywall Taping Knife about every 4 feet because the build-up will begin to smear on the ceiling after awhile, and I like very crisp, clean paint lines to give it a professional look.