Here is a design guide from Microchip describing low power design with PIC microcontrollers. The document provides theory behind some low power designs, and examples on how to implement them in your projects.
Low-power applications represent a significant portion of the future market for embedded systems. Every year, more designers are required to make designs portable, wireless and energy efficient. This document seeks to simplify the transition to low-power applications by providing a single location for the foundations of low-power design for embedded systems. The examples discussed in this document will focus on power consumption from the viewpoint of the microcontroller (MCU). As the brain of the application, the MCU typically consumes the most power and has the most control over the system power consumption.