Periodically clean the tracks where the sash must slide. Keep the tracks dirt and grease free, and spray them lightly with furniture wax to prevent sticking. Be sure to wipe off any excess wax.
Problem: Operating sash does not slide smoothly.
Solution: First, remove the operating sash. (See glider sash removal). Next, clean the sliding track by vacuuming and/or washing the surface with mild detergent. After drying thoroughly, wipe the sliding surface with a spray furniture wax. Then, reinstall the sash by angling the top of the sash into the upper track, then lowering the sash into the bottom track.