A plain bar of soap works well for reducing friction on wooden drawers, allowing them to move freely. Stay away from soaps that contain moisturizers and oils that can seep into the wood or gum up the sliders.
Pull the drawers out and turn them over. Wipe the sliders with mineral spirits if necessary to remove any grease or residue. Rub the sliders lightly with bar soap and place the drawers back in place. Move them back and forth. Apply a bit more soap if they still resist.
By keeping wooden drawers in good working condition, you'll reduce wear and tear, prolonging the life of the drawers. Choose a soap with a pleasing fragrance to give a fresh scent to old, wooden drawers, as well.
Silicone spray is a costlier, but effective, substitute for bar soap. Other options include paraffin wax or candle wax, although these products may cause sticking over time.