Tired of tugging open an obstinate double-hung window? You can usually smooth this sticky situation by widening the channels the sash slides in. The channels are formed by the vertical stop molding and parting bead — the strip located between the two sashes.
First, raise or lower both sashes. If the stops are thickly coated with paint, use a utility knife to score the joints where they meet the side jambs. Wearing safety goggles, hold a wood block against one of the stops and give it a few solid whacks with a hammer. Try moving the sash up and down. If it slides a little easier but is still tight, repeat this procedure on the opposite stop.