Scuff up the surface with steel wool. Rub a steel wool pad against the grain. This process will remove any existing finish and help create small lines of distress in the wood. If you are going for a slightly distressed look, this is typically sufficient.
Create dents and dings in the wood. Load up a canvas bag with marbles or even nuts and bolts. Tie the bag securely. Begin hitting the wood with the bag. You will notice dents and dings appearing at varying depths. Create a pattern that is pleasing to you and remember that distressing should be variable, not predictable. Move on to the next area after you are pleased with your initial starting point.
Sand down the distress marks. Sharp edges are not desirable in a distressed mantle. Go over all of your work with a fine grade of sandpaper to smooth out any rough spots. Wipe the mantle down with a damp cloth when you are done and allow the wood time to dry before proceeding.
Stain the wood. Choose your desired stain, preferably a lighter stain that will not hide the work you have just done. Choose an aged, gray stain for a rustic look or a light walnut stain for a more contemporary feel.
Seal the wood. Even though the mantle is now properly distressed, you do not want it aging or cracking beyond this point. Apply varnish or clear coat to the mantle to protect the wood against water rot and further aging.