How to prepare my home for vacation to reduce energy use?

The biggest thing by far is turning off/down heating and air conditioning.

Note that in some climates/jurisdictions, your insurance may not pay out for damage from any cause (not just frozen pipes) if the house wasn't heated while you were away. Mine says >15C at all times, which some parts aren't even overnight when I'm at home. I used to have a frost thermostat that satisfied a previous policy, but was incompatible with my more efficient heating system.

If you want to leave any lights on, use a timer, though they don't use much less power than modern LED lights.

If the heating is turned off in wintertime, the water-heater might be enough to keep the water pipes from freezing. Otherwise, the water can be turned off, the pipes drained with open faucets, and both heater and water-heater turned off. In fact the water might have to be turned off at the street because of the high-pressure it has coming from the street.

If the air-conditioning is turned off in summertime, several windows should be cracked open to avoid moisture build-up in the house. The window frames can be drilled and pegged slightly open. And I wonder about water in the toilets contributing to moisture build-up in the house.

For lights left on, the new 40-watt glass-bulb LED's at 450 lumens only use 4.5 watts. Well, LED's don't like too much heat while CFL's don't like too much cold.

Obviously, if the refrigerator can be emptied then turn it off.