If you have some antique sepia photographs from several generations back in your family's history, you can make frames out of rustic wood to display them. Mount the pictures on a cream matte background, and measure the dimensions of the matte. Order a piece of picture glass to cover it, and either nail the boards together around the opening, or miter the lumber to make a more traditional frame.
If you're building a set of shelves or cabinets from rustic wood for a more relaxed living area, adding some of your treasured photographs or artwork to the front of each door, and surrounding it with a "frame" of four pieces of rustic wood, makes the room seem more like your home.
Just because you're making a frame from rustic wood doesn't mean you have to restrict yourself to found lumber. If you can find some logs that are straight enough to make the edges of a picture frame, split a pair and you'll have four sides. You can either hot glue the matte to the logs, after you've put the picture in place, or you can use screws or tacks.
If you find an old metal sign from a granary or feed company and want to mount it on the side of your barn, or garage, throwing up some rustic lumber around the edges as a framing will help you transition from your more modern building to the idea of an early or middle 20th century feel.