5.1 Power Consumption
Because the Pi is meant as a low-power devices, and for some purposes you might even want to make it portable and power it using a battery instead of a power supply, here are some tips on how to make the Pi use as little power as possible. Most of this information was found here.
Disable HDMI
If you're using the Pi without a monitor, you can simple turn off HDMI output like so:
/usr/bin/tvservice -o
(use /usr/bin/tvservice -p to turn back on). Add the line to /etc/rc.local to make it permanent, otherwise it will only work until the next reboot.
Disabe LEDs
They only use a tiny bit each, but if every milliamp counts... check section 5.2 on how to power off the LEDs.
Minimize peripherals
Disconnect all peripherals you're not using, such as keyboards, mouse, webcam, etc.
Underclock
Run rpi-config to simply underclock the CPU.
Allow more power to USB devices
A.k.a. "My external hard drive keeps bugging out and I think it might be due to power issues"
The following will actually increase power consumption, but it might be useful nonetheless.
Some USB devices require more than the Pi delivers by default. E.g., a 2.5" external harddrive without its ow power supply will probably not work without the following fix (note: a 3.5" external drive will definitively not work without its own power supply).
The default output over USB is 600mA. If you are certain that you have a decent power supply that delivers at least 2A, you can increase the maximum USB output to 1.2A by adding the following lines to /boot/config.txt:
Raspberry Pi B and older
safe_mode_gpio=4Raspberry Pi B+ and Pi 2
max_usb_current=1Raspberry Pi 3
Already hardwired to give 1.2A by default. (source)