couple of ways 1) rub wax on the area you want to show through and when the top coat is dry buff off the top coat of paint leaving the waxed bottom coat showing through
2) use a harder paint on the base coat and sand off some of the top coat when it is dry
If you google 'Distressed paint effects' you should get loads more
If the wood's bare, you could wet it. This will make the wood swell slightly, so the grain will stand out more once it's dry. If you're going to sand after applying the paint, I'd thin it. Less clogging of the sandpaper.