The easiest technique, painting and sanding, works best on dark wood frames. Rub the original frame lightly with sandpaper to roughen the surface, then paint it with colored paint. When the first coat is dry, apply a second coat of paint in a different color. Finally, apply a third coat in a different color. This top coat will be the most visible, so choose a color that ties in with your decor.
When the top coat is dry, take some fine sandpaper and very carefully sand off the layers of paint at the places that would receive the most wear, such as the corners and edges. Sand through until you see the original dark wood underneath. Distress the main body of the frame by hitting it with a light chain, or putting it face down into gravel and rocking it gently, having first removed any picture glass.
You can use the waxing technique on any type of frame, including pale wood and even plaster, as it's gentler than the painting and sanding technique. Paint the frame in a base color, let it dry, then rub random areas with candle wax. Apply another coat of paint in a different color. When the second coat is dry, use a soft cloth to rub away the paint from the waxed areas, where it won't have adhered properly. To deepen the effect, apply more wax and then a third coat of paint.
Crackle glazing is a technique that mimics the glaze on antique furniture, where the original varnish has cracked with age. It's a simple effect to achieve yourself, and looks more elegant than a heavily distressed finish, which will have a rustic look. Paint the frame with a base coat of paint in the color of your choice. When the paint is dry, apply a thin layer of oil-based varnish. When the varnish is still slightly tacky, apply a layer of water-based varnish. The water-based varnish will dry more quickly than the oil layer, and cause the surface to crack and divide, giving an attractive, textured finish.