I'm in the middle of a fairly big painting job at the moment, repainting the window frames and doors of the grange. I like to do a long job like that in small steps, and I'm doing one window or door per day, weather permitting. That way I don't get too bored, and I can keep my attention focused on the job for the necessary time so I make fewer mistakes.
The paint is oil-based, or at least you have to use spirits rather than water to clean the brush, and the paint is immiscible with water. So rather than use brush cleaner at the end of each day, I just dunk the brush in a jam jar of water. That way the paint doesn't dry out, and the brush is ready for use the next day after a quick wipe with a newspaper. Thanks to my immensely talented brother-in-law Julian for this tip.
P.S. What if the paint is water-soluble? Wrap the brush in cling-film.