Power Saving Issues

Sometimes, unfortunately, you must run on batteries
Every milliamp (or even microamp) is important
Fortunately, this doesn't apply to alway-on acquisition devices

Working on the microcontroller
Lowering the frequency (power is rated in W/MHz)
Cons: effects on performance but also pwm, spi, uart, ...
Lowering the voltage (most devices run down to 1.8V)
Cons: I/O voltage issues with peripherals
Entering sleep/deep-sleep modes
Cons: dealing with wake-up in sw

Working on the power supplies
Evaluate switching over linear (but consider costs)
Consider the indivdual chips (the popular 7805 draws 6mA!)

Working on peripherals
You can turn off all peripherals, if possible