Most spices are available whole, ground and powdered. Ground spices lose their flavour and aroma quickly so buy them in small quantities and store in airtight containers in a cool, dark place for up to 6 months only.
Similarly, spice mixes will only keep for a short time and the individual spices may have different rates of staling, which can upset the balance of flavour.
Where possible, buy spices whole and use your own spice grinder or mortar and pestle to grind them as needed.
Buy good-quality spice mixes — those that contain whole spices should be in good condition and not just a way of using up broken spices; powdered mixes should be freshly ground, if possible.