If you lack confidence in avoiding getting paint onto surfaces you don’t want to paint, use quality masking tapes to “mask off” these areas.
If, for example you have painted your ceilings and you are about to start your walls but you are concerned you might get wall paint onto the ceiling, you could carefully put masking tape across the bottom lip of the ceiling cornice where it meets the wall, or if you are painting door frames or window edges and you don’t want to get paint onto the walls, you could “mask off” the walls.
By using quality masking tape, which is usually blue, green or even purple, you are ensuring the tape is of a quality that will not allow paint to penetrate through it, will give you a nice straight line when it is removed and will not peel off paint from the surfaces it is applied to.
Even freshly painted surfaces that are “dry” can have “masking tape for delicate surfaces” applied to them without having the fresh paint peel off. When you have finished painting, peel the masking tape off while the paint is still “wet” to avoid getting a dry film between the two surfaces.