There are many methods for distressing lumber to give it an aged and "well-loved" appearance. Beyond the step mentioned in the beginning of selecting boards with imperfections, you can essentially "beat up" a project.
Use a scrap length of chain to add dents and nicks. For deeper dents, use a hammer (Images 1 and 2).
To create the look of wormholes, use an awl or ice pick as shown in Image 3. Try creating holes in different angles as well.
Create worn or chipped edges by cutting notches using a pocket knife or utility knife as shown in Image 4.
Wear down random spots using a sander as shown in Image 5.
It's hard to give a weathered appearance to new boards, such as the lumber we use here, but you can cheat that gray patina by diluting dark gray paint and applying it as you would a stain as shown in Image 6. Add water and the paint to a quart-size container and test the finish on pieces of scrap cut from this project until you get the look you desire.
Apply a coat of polyurethane when finished.