Put on safety glasses and gloves. Clean the shovel handle with a stiff-bristled brush to remove dirt and debris, especially in the places on the handle where you grip it when digging.
Sand the length of the handle with medium-grit sandpaper until the handle is smooth. Check the surface for any remaining splinters, and sand them off. Wipe down the handle with a tack cloth to remove any particles from sanding.
Examine the handle for any cracks. If the handle has split, clean the surfaces where the split occurred with a toothbrush, and don't sand them. Brush each split surface with a small paintbrush dipped in yellow wood glue. Clamp the handle back together with three or four wood clamps, and let it dry for at least an hour. Fill any surface cracks with yellow wood glue. Remove the wood clamps when the glue has dried.
Sand the handle again with medium-grit sandpaper to remove any excess glue, and wipe the handle down with a tack cloth, removing any bits of sanding debris.
Soak a rag in boiled linseed oil. Coat the shovel handle surface with the oil, and let it soak into the wood. Coat the handle several more times, letting it sit in between coatings. Wipe off any excess oil with a clean rag. Spray the areas on the shovel handle where you grip the most with a spray-on rubber coating to protect the handle.