Terracotta does not crack because of the cold. It cracks because water gets into the clay, freezes, and expands with nowhere to go. Everything below follows from that one fact, and it is why a pot can survive a hard winter in one spot and shatter in another twelve feet away.
1. Empty the saucers
A saucer full of water is a pot standing in ice. The base wicks moisture up into the clay, the clay freezes, and the bottom third splits away. If you take one step from this list, take this one — tip every saucer out and, ideally, put them away until spring.
2. Lift pots off the ground
Pot feet, bricks, or a couple of offcuts of timber. Raising a pot even an inch lets the drainage hole actually drain instead of sealing itself against wet paving. It also stops the base sitting in the sheet of ice that forms on a patio overnight.
3. Move the porous ones indoors
Unglazed terracotta and low-fired clay are the first to go, because they absorb the most water. Glazed ceramic is better but not immune — the glaze only covers the outside. Concrete, fibreglass and metal generally stay out without trouble.
A garage, a shed, or even against a sheltered house wall is enough. It does not need to be heated; it needs to be dry.
4. Group the rest together
Pots clustered against a wall hold warmth far better than pots scattered across a patio. You are creating a small thermal mass — each pot shelters the next, and the wall radiates back what it absorbed during the day.
Put the most vulnerable ones in the middle of the group and the toughest on the outside.
5. Mulch the tops
Two inches of bark, straw or leaf mould over the soil surface. This insulates the root ball, which is what you are actually trying to protect, and it slows the freeze-thaw cycling that does the most damage to both roots and pot.
6. Water sparingly
Counter-intuitive, but: dry roots survive cold, wet roots rot. Most plants in pots are killed over winter by waterlogging rather than by frost. Check occasionally, water only if the compost has gone bone dry several inches down, and never water when a freeze is forecast in the next day or two.
What to do if one cracks anyway
A hairline crack in a pot that is otherwise sound will usually hold for another season if you keep it dry and out of the freeze-thaw cycle. A pot that has split through is finished as a container but makes a good crocking layer in the bottom of the next one.
If you are replacing, glazed and cement planters handle winter considerably better than raw terracotta — and the large ones do more for a patio corner than several small ones ever will.