Thaumaturgic foci would be things you'd use regularly in every spell of a given type--for example, a Seer in one of my games had a crystal ball she used for divination control. Most of my own characters who have them are for crafting (one has work gloves, another has a wrench--things they'd use to craft each item).
So, think of Foci as the things you keep in a tool box (hammer, screwdriver, vice grip, crowbar), while the declarations and maneuvers you make are the disposable bits (Nail, screws, staples, bits of wood).
As a rule of thumb: If you are going to use it for a given type of spell? Focus. If it's only for this spell? Declaration.