Remove the curtains from the rod. Remove the rod from the window, if possible, to make it easier to work on.
Rub the stub of a candle over the top of your curtain rod. This deposits a fine layer of candle wax along the top of the rod.
Slip the curtain rings over the rod once more and remount the rod over the window. The rings should glide over the candle wax.
Remove the curtains from the rod, and take the rod down from over the window. Sand the rod lightly with fine-grain sandpaper.
Wipe the sandpaper residue from the rod. Paint the rod with two coats of high-gloss enamel paint. Let it dry.
Remount the curtain rod over the window. The enamel paint provides a slick surface for your curtain rings to glide across.