If you are using alkyd paint or a finish that requires thinner in the cleanup process, choose brushes with natural bristles. If you are applying latex paint or a finish that is water-based, choose brushes with synthetic bristles as natural bristles lose their integrity when wet.
Brushes made from polyester not only keep their shape in water, but they also hold up to humidity and heat. Brushes made with nylon will not wear out as readily, but they do not hold up as well when the temperature climbs.
If you are painting an average room, all you should need is a 4-inch brush with a straight edge, a 1 1/2-inch brush for sashes, and a 2-inch brush for trim.
Before purchasing, hold the brush as you would when painting. Make sure it is a comfortable weight. Fan the bristles to make sure there are no gaps, and “bounce” the tip of the brush to test for springiness.