Buy dried, whole or ground from a specialised spice store or a store where you are assured of a high turnover of product.
Store in an airtight container in the pantry. Whole cloves, stored properly, will last for many years. Ground cloves are at their best for about 15 months after purchase.
Tips & Tricks:
Cloves traditionally marry with apples, but can be used in small quantities in sweet spice mixes and curries. To rid the house of an unpleasant smell lay a couple of cloves on the electric ring of the hotplate and turn the rings to a low heat. This won't work with gas, of course. Biting on a clove will release a numbing oil which can be used as temporary toothache relief.
Cloves are delicious in sweet spice blends for fruit cakes, pickles, preserved meats and curries.