In general, the job of painting windows will go faster if you purchase a 2-or 21/2-inch sash trim brush, angled slightly across the bottom to make it easier to get into 90-degree corners and tight spaces.
To paint wood-frame windows, first raise the bottom sash more than halfway up and lower the top sash until its bottom rail is several inches below the bottom sash. Paint the bottom rail of the top sash and up the stiles as far as you can go. Paint all the surfaces of the bottom sash except the top edge. Reverse the position of the sashes: top sash up to within an inch of the window frame, bottom sash down to within an inch of the windowsill. Then, paint the formerly obstructed surfaces of the top sash and the top edges of both sashes.
Don't paint the wood jambs in which the sashes move up and down yet. Instead, paint the window frame, working from top to bottom, including the sill. When the paint on the sashes is dry to the touch, move them both down as far as they will go. Paint the exposed jambs. Let the paint dry, raise both sashes all the way, and paint the lower jambs. To keep the sashes from sticking in the jambs, put on only as much paint as is necessary to cover the old coat. Wait for the paint to dry, then lubricate the channels with paraffin or a silicone spray.