When buying spinach choose bright, bouncy-fresh leaves and remember that it cooks down to almost nothing, so you always need much more than you expect. Store in a cool, dark place and use within two days. Large-leafed spinach sold loose needs a thorough wash to remove dirt and grit. Discard tough stalks or any damaged leaves.
Supermarkets now sell bags of tender young spinach leaves which, though pricy, have the advantage of not needing much preparation. Baby spinach is sold at a premium to use in salads. However, size isn’t necessarily a guide to quality; large leaves can be succulent and full flavoured and are usually the best bet for cooking. Frozen spinach is best reserved for emergencies, but is also good used in stuffings and fillings.