I found something on the XDA forums the other day. Apparently it doubles your battery life, this method comes directly from HTC, but it was talking about the HTC Incredible, people on the XDA forums have reported it works on the EVO and Desire S:
"To also help with Battery Life you can do these steps exactly:
1) Turn your device ON and Charge the device for 8 hours or more
2) Unplug the device and Turn the phone OFF and charge for 1 hour
3) Unplug the device Turn ON wait 2 minutes and Turn OFF and charge for another hour.
Your battery life should almost double, we have tested this on our devices and other agents have seen a major difference as well."