If your furniture piece is already painted, you can still find a way to show the look of natural wear. To do this, first look over the piece and consider which areas are more likely to receive excess wear. Areas such as knobs, handles, edges and any other places where contact with hands would be frequent are good candidates. Along these areas, you'll want to scuff the piece with sandpaper of any grade. Leave deep impressions by pressing the sandpaper hard or rub the target areas lightly to create small scuffs. Dust off any debris and stain or seal the piece to lock in the look of years of use.
To achieve a similar effect on an unfinished piece of furniture, sand the surface and apply a primer. Paint a base coat over the piece and allow time to dry. This color will show through, representing wear, so often a deeper color is recommended; however, you can use any color you wish to show through on those high-use areas. Rub small amounts of Vaseline with your finger along the areas where you want the base coat to show through. Paint over the entire surface of the piece with a second color of paint, covering it completely and let the paint dry. The top coat will not adhere to the Vaseline, but will attach to the base coat. Use steel wool to rub the areas where you had applied the Vaseline to easily reveal the base coat underneath.
To create the look of old paint that has split and now shows the original paint underneath, you'll want to add what's called a "crackle finish." To begin, sand the piece if necessary and wipe away any excess dust with a damp cloth. Paint on a base coat of acrylic paint to the furniture. This paint will be what you want to see showing through the small cracks. Let the piece dry completely.
Apply a crackle medium to the piece over the base coat. To create small cracks spread the crackle medium thinly, but for larger cracks, use a thick layer of medium. Within 2 to 4 hours to apply your top coat of acrylic paint. The top coat should be a different color than the base coat, as it is the one that cracks to show the base. The crackle medium works best if the top coat is applied in long strokes without overlapping previous strokes. When you are finished painting, let the piece dry. As the piece dries you'll be able to see the crackle finish already beginning to appear.