I keep all my tools sharpened. Work goes quicker and easier with sharp tools.
A shovel with a curved blade depends on its edge to easily penetrate tough soil, roots and other materials. Keep this edge sharp with regular cleaning and sharpening. Use a large file and match the angle of the file with the bevel on the blade, generally about a 30 degree angle to help the shovel keep its edge.
For the blade of my scythe I use sharpening stones to maintain a keen edge.
For details seethis article on Mother Earth News.
During the off season I mix Linseed oil and paint thinner in equal parts and apply several light coats to the wooden handles of my tools. Linseed oil dries very slowly so I do this in warmer weather in a dry area.