1. Gather cohesive pieces for a more formal style....keep the same color in all of the compositions, or use the same frames with different photos or art. You can even stick to a theme of the same style art. Don't forget the use of a tryptic -A series of artworks that are meant to be put together with distance between them. This is a great look for symmetry, and formality,
2. Mixing large and small pieces on the same wall is a good idea so your eye can focus on multiple areas on the wall . Depending upon the sizes of art, I often hang one large piece first then work everything else around it.
3. Spacing matters. I often keep spacing the same between all frames sometimes hanging small two matched frames over a large one for a symmetric look. Keep the thickness of the frame in mind, since you'll sometimes need to tighten up the hanging so that frame looks balanced.
4. Using the same sized frame creates elegance while the art can be unique and different in each one but don't be afraid to mix compositions and styles if you want interest and your personality to shine.
5. You can absolutely mix color and black-and-white, keep your frames consistent and it all works.
6. Don't hesitate to mix shapes and elements. An architectural element or favored platter can work well together.
7. Be sure to incorporate your existing furniture and lighting. A pair of symmetric lamps frames this gallery, and assymertric art stacked or grouped on a wall can balance a large curio or armoire.