Asymmetry: If you want to mix and match, add itms of the same color pallate. For instance, if you have a colored candles sticks, try to add the same color flowers in a neutral vase on the other side. Fee free to bunch "like" items together to create clusters!
Pattern and Texture: By doing a runner in a fun pattern or texture, you give nice contrast between it and the exposed wood/glass below!
A themed collection: like items from nature, makes a statement about what’s important to you and is tied together visually. Ceramic and glass are a nice counterpoint to rough pieces. Vary tall and squat, smooth and rough, flat and three-dimensional...think textures!
Layering: pictures and post-cards creates a casual, unfussy quality, inviting investigation rather than scaring people away.