Buy spices preferably from a specialist spice store where you can be sure of a quick turnaround of produce and guaranteed freshness.
Store in an airtight container away from direct sunlight or in the fridge. Try to use freshly ground curry powder straight away.
Tips & Tricks:
Make your own curry powder - it's more fun and will be fresher than anything bought from the supermarket.
Dry roast the following: 1 tsp black peppercorns, 1 tsp mustard seeds, 1/2 tsp coriander seeds, 1/2 tsp cumin seeds, 1/2 tsp fennel seeds.
Add 1/2 tsp cayenne pepper, 1 large bay leaf, 8 whole cloves, and 6-12 cardamom seeds (pods removed).
Grind all the ingredients together until fine.