Dogs and wood floors are not the best combination; inevitably their nails will scratch and dent the floor, and the bigger the dog, the bigger the scratches. Maintaining mirror-like perfection on your wood floor finish is probably an unattainable goal if you have a dog, even if you trim its nails short regularly. If you routinely repair the scratches, however, you can keep that homey, lived-in look without degenerating to having utterly ruined floors.
Assess the depth of the scratches. If they are surface scratches and have not penetrated through to the wood or dented the floor, they'll be easier to repair. If the scratches are deep and into the wood, you can significantly diminish their appearance, but repairing them is a little more involved.
Use a wax stick or paste wax for surface scratches. Wax is available in clear for lighter floors and in various shades of stain for dark floors.
Rub the wax into the scratches with a soft rag or fine steel or brass wool, available at hardware stores. Rub it in well, working along the grain of the wood.
Buff the wax with a dry rag until it is blended in with the surrounding floor. If the scratches still show up, reapply wax and buff again.
Saturate deeper scratches that have penetrated through the wood with rubbing alcohol. Lay a damp cloth over the scratch, and run a hot iron across it. If the wood fibres have not been too damaged, the moisture from the rubbing alcohol and damp rag will swell the fibres when you apply heat. Let the scratch dry, and apply the wax as in the above steps.
Use carpenter's wood putty to fill deep scratches that don't respond to the rubbing alcohol treatment. Use a 25 mm (1-inch) flexible putty knife to apply it to the scratches. Scrape away excess so you'll only have to do minimal sanding. When dry, sand it carefully with 120-grit sandpaper or steel wool, avoiding the surrounding wood as best you can. Wax and buff it as above.
Keep your dog's nails trimmed short and smooth. Note areas of the floor that frequently get scratched and lay non-skid rugs down. These areas might include doorways, around the food bowl and by windows. If you have a newer urethane-surface finished floor, purchase a scratch repair kit from a flooring dealer.