1. Shutdown and disconnect power.
2. Remove battery.
3. Hold down power button for more than 5 seconds.
4. Reinsert battery and reconnect power.
Detailed information on resetting the PMU on MacBook and MacBook Pros is available via Apple's Support site. Resetting the PMU should only be considered in last-case scenarios, and should not be done more than once every 48 hours.
Disclaimer: Users should proceed at their own risk, as MacRumors cannot be held liable for damages that may be caused by following the instructions listed above. MacRumors has not independently tested the above procedure as such resources are unavailable, but the tip is rather offered as a courtesy. Users are strongly encouraged to contact their AppleCare support representative and consult Apple's documentation for guidance if following the aforementioned steps.