The sill is the part of the frame that takes the worst beating because it extends out from the window.
To protect a particularly vulnerable sill, you can cover it with a sheet of aluminum and paint it to match the wood. This is actually a very easy project and offers excellent protection.
» Trim sheet aluminum with tin snips
First, cut a paper template that fits the sill. Lay the template on sheet aluminum (available in rolls or sheets at home improvement centers), and mark the pattern on the metal.
Using tin snips, cut out the pattern. Caulk the sill edges and wedge one side of the aluminum to the stool of the sill and nail it in place with 1-inch roofing nails.
» Cover the area needing protection, then paint
Tape a block of wood over the surface of the aluminum and tap it with a hammer so the aluminum takes on the shape of the sill.
Nail it underneath the sill and seal it along the edges with caulking compound. Prime with metal primer and paint to match your trim.