Buy a Good-Quality, Angled Sash Brush
We've seen many pro painters cut in perfectly with a big square-edge brush, but for most people, angled sash brushes, like the ones shown here, are easier to control. You can fan the angled tips out to get a fine line of paint, and the angle makes it easier to get into corners. A 2-1/2-in.-wide brush is about right for most interior room painting. But if you're painting windows or other small woodwork, buy a smaller brush too. It's easier to paint accurately with a smaller brush.