Buy cumin seeds, not the ground cumin. Warm the seeds briefly in a dry skillet and grind them as needed.
To roast cumin seeds, heat a small heavy skillet over a medium-high flame and then add the amount of cumin seed called for. Stir it gently, and when it begins to pop a little, be sure not to let it turn black. Dump it from the pan, let it cool a little, and then grind it in a coffee grinder. You can even make a larger amount and store it for up to two weeks in your spice cupboard.