Diagnosis
Dust buildup between the metal fins is the main maintenance issue for these. It insulates the fins, making the heater less efficient, and can eventually become a fire hazard if it's thick enough.
How to Fix It
Unplug the heater and let it cool completely for several hours. Use the brush attachment on your vacuum cleaner to gently suck dust from between each fin. For stuck-on dust, use a dry, soft paintbrush or a can of compressed air (used outdoors or in a well-ventilated area) to blow the dust out from the back towards the front, then vacuum it up.
If That Didn't Work
Never use water, liquid cleaners, or a damp cloth on the fins or inside the unit. For grease or grime that won't come off with dry methods, you can very lightly wipe the exterior metal surfaces with a cloth barely dampened with mild soapy water, then immediately dry it. Keep all liquid away from the electrical components, control panel, and base.
Heads Up
Always unplug the heater and ensure it's cold before cleaning. Cleaning a warm or plugged-in unit risks electric shock and can cause the metal to warp if you use a damp cloth.