Lightly sand the edges of the stair treads and risers with a fine-grit foam sanding block. Use just enough force to remove the finish from edges and raised areas of carved designs.
Sand the center of each tread, where normal footfalls occur. Begin with a large section, sanding side-to-side, gradually narrowing your sweep until you are concentrating on the center four to six inches. This mimics the look of heavy use, with bare wood in the center that gradually fades to the regular finish on the seldom-used sides. Stop the process before you reach bare wood if you find the look you want.
Sand the center front edges of the treads with coarse sandpaper to rough up the wood and give it the fuzzy look that comes with years of use. Aim to "fuzz" an area the width of the bare wood in the center. Wiping the edges with a damp rag after sanding makes the wood grain expand for more obvious fuzz.
Beat the treads with a heavy chain for a randomly scarred look. The scars will eventually darken and be more obvious than they are at first, so don't overdo it. Use a different swing for each tread to avoid a cookie-cutter look.
Hammer several large nails through a piece of 1-inch by 4-inch wood about 16 inches long or so. Concentrate the nails toward the top of the board in a random pattern until it looks somewhat like a hair brush. Use this tool to beat the stair treads for obvious distress scars.
Repeat the distressing process until you achieve the level of ruin you're looking for. Keep in mind that additional wear will occur over time. Don't ruin the structure of the stair case.
Rub any areas of bare wood with tung oil to provide moisture protection without altering the bare wood look.