Sleep modes are your friend. Generally you want to put the chip into the lowest level of sleep possible depending on your requirements for wake up while you aren't doing anything. You also want to clock as slow as possible to complete your tasks within whatever timeline you have. As Nigel said, if you want to go that far, you can adjust clock speed on the fly. I do this on a project with an LCD. Clock as fast as possible to clear or write to the LCD so the LCD doesn't lag, and clock slower when working with other peripherals like the radio.
Someone did the math a while ago about doing everything as quickly as possible and sleeping, or clocking slowly and sleeping and the clocking slowly won out. Not an EE and the math hurts my head, so no details unfortunately.
Generally newer chips have ridiculously low power requirements if you spend a bit of time dropping into sleep or even complete shut down any time you aren't processing.