What is the best way to cut in when you are painting a wall?

I find a 2 inch angular paint brush works best along ceilings and around wood work. Cut in a little at a time so when you roll the rest, you don't have lap marks.What is the best way to cut in when you are painting a wall?
ask the wall if you mind that you dance with her partner.What is the best way to cut in when you are painting a wall?
i learned a cool trick from a painter when switching colors on walls, but nothing to seperate, like on a round corner, tape a line where you want it to end, then caulk on the edge of the line you are going to paint and wipe it, just like normal, then paint. it is a perfect line and no bleed thru since the caulking stops it.