I turn the main water valve off, turn the gas water heater to pilot,
and turn the heat down to 45. How low you can safely go on the heat
depends on the weather, the home construction, the type of heat,
location of pipes, etc.

An example of how trouble can occur. I was away for a week and had
done the above. Upon returning, when I turned the water back on, there
was now a leak coming from the plastic nut on the bottom of the toilet
tank, where the ballcock meets the riser supply line. Apparently, the
change in temp as the house cooled off was enough to cause it to begin
to leak. Had the water been on and just the temp set way back, I
would have returned to what could have been a real problem.