Use the Rule of Odds
In a composition, people are more attracted to odd numbers than even ones. Objects arranged in odd numbers are more appealing, memorable, and effective than even-numbered groupings because your eye is forced to move around, which makes for a more interesting visual experience. So try using five frames instead of four, or three instead of two.
Complement Your Art
Picture frames and mat boards should always support, not compete with, the art. Choose a color present in the artwork to make it pop, like a beige frame to draw the eye to a bit of sand in an ocean scene. Or stay within the same aesthetic as the work. A Baroque-style painting, for example, would look right at home in an ornate gold frame, while modern art is more well-suited for a sleek metal frame.