Verify that all window and door seals/weatherstriping is in good shape.

Check to be sure doors and windows are caulked properly (no gaps, cracks or missing).

If you have an attic access, treat it like door opening and seal properly. If possible, create an insulated cover to go over it in the attic.

Check HVAC ductwork for leaks and seal with foil tape or mastic.

Wrap your hot water heater in a blanket.

Use a tester to verify the power is off before starting repair. Use pliers with insulated handles. Wrap electrical tape around switch or outlet screws before installing into box. Make sure all devises are grounded, including metal boxes.

You can also look into programmable thermostats!

I would start with making sure your attic has adequate ventilation.