How do you achieve a weathered look when painting wood?

There are several things you can do. What look, specifically, are you hoping to achieve? An antique? Rustic? Beat up? Or just faded?

Paint in several different colors allow each layer of paint to dry, then sand at the edges and anywhere normal wear & tear would occur.

I can assist further if you give more examples.

some furniture places actually beat the wood with a big hunk of chain, and then paint over the dents when they paint the whole thing. I made a surgical tool cabinet out of a glass faced wooden box with a hinged lid, and I used files, drill bits, and Exacto knives to make chips, cuts, and worm holes in the case before I painted it. It turned out pretty good. @SpatzieLover gave you some good points, too.