Dust off the wood with a high-quality duster, such as one made of wool, and then blow dust out of the grooves with compressed air in a can, such as the kind designed for cleaning computer keyboards.
Dampen a sponge or cloth in a good-quality wood cleaner mixed with water, and wipe down the wood. Start at the top and work your way down.
Wipe the wood down with a towel to dry it well, and then apply a coating of lemon oil if the cleaning restored the wood's luster. If the wood still needs work, you may need to apply a new coat of varnish or shellac.
Fill scratches and nicks with wood filler. Small scratches can be filled with matching shoe polish or even iodine applied with a cotton swab.
Sand the wood so the former finish comes off and the scratches and nicks are even with the rest of the wood.
Use a tack cloth to remove any dust from the walls after sanding.
Apply the new finish with a paintbrush or roller, according to the manufacturer's instructions.
Allow the finish to dry, and then maintain the walls on a regular basis with cleaning as described in Steps 1 through 3.