First you need to wash the potatoes and dry them very thoroughly with a cloth, then leave them aside to dry as much as possible.If you're using ready-washed potatoes you need not do this, as the high heat will purify them. Next, prick the skins a few times with a fork, then put a few drops of olive oil over each one and rub it all over the skin. After that, rub in some crushed salt – this will draw out the moisture from the skin and, together with the oil, provide more crunchiness.Now place the potatoes straight on to the centre shelf of the oven and let them bake for 1¾-2 hours, or until the skins are really crisp. When you are ready to serve, slit each potato in half lengthways and top with the butter and seasoning. Serve immediately because, after you remove jacket potatoes from the oven, they lose their crispness very quickly, so don't let them hang around.