Always try to seal the room(s) from escaping heat. Especially HVAC vents, lights, bathroom exhaust fans, bottoms of doors, wall air-conditioners, etc… Cover windows pane with blankets or equivalent to minimize loss of energy from cold windows.
Remove items that you would not leave in your car on a hot sunny summer day. Candles, lighters, many foods, candies, etc…
Unplug all electrical items and do not place electrical items directly in front of the heater. Electrical items are rated for higher temperatures than you are heating, just avoid the direct heat coming out of the heater.
It is best to bag clothes, sheets, etc… and run in the clothes dryer for 20 minutes on high. Never leave clothes laying on the floor or other surfaces or they will act as an insulator and it will be difficult to heat under them sufficiently.
Open all drawers and separate all items so hot air can get everywhere easily.