Rub a painted table with steel wool. Concentrate on the areas that would naturally receive the most wear: the edges, the corners and any raised designs. Sand until the wood begins to show through. If you wish to remove part of the wood, use sandpaper--or even an electric sander--before you use the steel wool.
Rub unpainted furniture the same way. This will round the edges and lighten the finish in these areas.
Use a length of chain to beat dents into the furniture. Again, concentrate on the areas that would naturally receive the most wear: the top and the edges of the top. If your table has a pedestal base, put lots of wear marks on the top of the base, as if generations of people have rested their shoes there.
Lay a screw on the table and pound it with a hammer a few times for an interesting dent pattern. Repeat this in another spot with a different-sized screw. Twice is enough--you do not want to make a pattern. Add a few random hammer dings.
Place hot pots and pans on top of a plain wood kitchen table to make burn rings. Most people do not want to damage their tables this much, but it will give an old farm table a very authentic look.
Rub the distressed areas with steel wool to remove any splinters. Finish with several coats of beeswax or furniture polish.