A few years back we went with PIR sensors for all of our office space and other spaces lighting. If I exit my office the lights turn off after I think 10 min. When I enter they automatically turn on. We also went with new high efficiency flourescent tubes. The cost savings were about 50% and the system has just about paid for itself.
On another note, our HVAC does exactly what BrownOut mentions. Monitor Outside & Inside air temperature and meters the outsirde air in accordingly. That works real well and an added nice feature is the fresh air!
Ron
That "feature" is often called an "economizer".
All you need is a actuator controlled damper. most digital thermostats have the option to control them.
Making your own is pretty easy too.
A uC with a thermometer and a relay or h-bridge to control the actuator. (Or a logic controlled actuator)
You tell the uC the "requested" temperature in the room/building
If the outdoor temperature is <= (less than or equal to) the requested temperature, it opens the damper.
The damper/actuator is located on the air handler and a duct from the damper to the outdoors is added. A screen and air filter are usually added. The duct size is dependent on the system size, but even an undersized economizer duct will save money/energy and slow the cycle on time of the A/C.
This is a great thing for server rooms, or any place where machinery raises the rooms temperature above a "happy" point, even during winter.
In winter, it seemingly makes no sense to use the A/C, because it is so cold outside.
The reason behind it is, the A/C removes moisture, so in cases where humidity needs to be controlled, adding a de-humidifier to the economizer is a good idea.
That or you can make sure the economizer duct intake is before the A/C coil in the air handler. That way the coil will help remove excess moisture.