Paint Thinner $$$

This 1/4 oz. bottle of Testor's Thinner and Brush Cleaner sells on Ebay for $2.79 plus shipping! Very expensive when you consider how long it will last.

Most builders will dip a wet brush and swish it around in the thinner bottle. This leaves too much paint in the thinner that will end up on your next brush.
DON'T MASH THE BRISTLES ON THE BOTTOM OF THE BOTTLE! That will break and bend the bristles.
You are better off just dipping the brush in (don't swish) withdraw the brush and wipe on a paper towel. Repeat until the brush is clean.

You could dedicate one brush to each color as it is hard to remove all the paint left in the bristles. If you later use a "red" brush with white paint, your touch up could have a pink tinge.

You can also clean a brush pretty well using an acetone based nail polish remover. Try to buy 100% acetone remover.
Dip in the brush, remove and gently wipe off on a paper towel. Repeat until you no longer see any paint being transferred onto the paper towel. Just dipping the brush in then removing it will keep the acetone clean.

This Acetone clean is only for natural hair bristles. Acetone will melt the cheapest plastic brushes.

This follow-up I learned from my Mother, she was an artist, sometimes painting in oil.

After the acetone clean, place a drop of liquid dish washing soap in the palm of your hand. Add a few drops of water.
Gently swirl the brush in the soap and water. You might be surprised how much remaining paint will come off the brush.

NOTE: I use a dedicated brush for each color. I probably have ten good brushes, each marked for a specific color.