You will get the biggest impact on energy usage if you understand what is using most of the energy and focus on that first. It makes no sense to optimize your lights if your HVAC is consuming 80% of your total energy usage.
Beyond that what you can do will depend on the situation. There have already been several ideas mentioned here which boil down to these options:
- only power a device when it’s needed
- lower/raise the setting on the device so it consumes a little less energy
- use other means to achieve the same goal (e.g. rollershades to avoid the heat rather than solely relying on the AC, improving the insulation, etc.)
- replace the device with one that consumes less power (note, replacing a perfectly fine and working device with a new one may have negative ecological impacts that overwhelm the impact of the energy savings, new is not always better all things considered).
In my experience, the third option usually has the biggest impact over all compared to the others. But at that point you are outside the realm of automation and into the realm of good building practices and renovation.
In the realm of automation, good presence detection is going to be key. If you have fine control over your house on a room by room basis (e.g. each room has it’s own climate control, lighting, etc) then the more fine grained the presence detection the better (i.e. down to which room is occupied). Though I suspect this leads to diminishing returns for the amount of effort expended.