Start with one tall piece
A vase or lamp gives the shelf a vertical line. Everything else can sit lower without looking flat.
Leave air
Empty shelf is not unfinished. Negative space lets clay and glass read as objects instead of clutter.
Mix use and display
A stack of books, a bowl that holds keys, a candle you actually light. Useful pieces keep the shelf from feeling like a showroom.
Repeat a material
Two clay pieces and one glass object will feel calmer than five unrelated materials. Echo a tone or texture once across the run.
Edit once a season
Remove something before you add something. That habit alone keeps the shelf quiet.