Material
The cheapest protection there is, and one that most traditions arrived at independently.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Salt is sodium chloride, halite when it comes out of the ground, and its magical reputation follows its chemistry almost exactly. It preserves meat, kills slugs, draws water out of whatever it touches and does not rot. A substance that stops decay was always going to end up in the protection cupboard, and it did so more or less everywhere people had access to it.
Lines of salt at thresholds, salt carried in a pocket, salt thrown over the shoulder, salt set on a corpse — all of these are widely recorded across Britain, Ireland and Europe, and comparable customs turn up far beyond. Its being cheap enough for everyone and valuable enough to tax gave it a double character it has never lost.
Practically, use it with some care. Salt kills plants and corrodes metal, so lines belong on hard indoor surfaces rather than on soil, doorsteps you value, or anything with a paint finish. Black salt, which is ordinary salt blended with ash or charcoal, is a working material and not food.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
Common in contemporary practice
A line at a threshold, swept up and replaced rather than left indefinitely. Most practitioners treat used salt as having taken something on, and dispose of it outside rather than back in the box.
Carried forward
John Brand, Observations on Popular Antiquities (1777)
Brand records the plate of salt set on the breast of a corpse in northern England and in Ireland, and notes that the people keeping the custom gave quite different reasons for it — some practical, about swelling, and some not practical at all. He declines to choose between them, which is the correct instinct.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.