Dirt and dust absorb heat and reduce heat output. Since radiator covers block airflow, you need to remove them when the radiators are in use. Placing radiator reflectors on the wall behind the radiator unit increases the amount of heat radiated back into the room. This is an inexpensive scoop-shaped device that attaches over the register, typically with magnets. If your main door opens to either an unheated hallway or directly to the outdoors, make your door airtight.
Warm air can escape between the doorframe and the wall.
If this is the case in your apartment, caulk between the frame and the wall. The loss of warm air is frequently greatest under the bottom of the door. Prevent this by installing a door sweep on the bottom edge of your door. A low-cost alternative to a door sweep is a draft guard. This is a closed tube of cloth filled with sand that is laid against the bottom of your door.