Check and change the oil. No single step will help an engine last more than regular oil and filter changes will. Conversely, nothing will destroy an engine faster than neglecting oil-level checks or fresh-oil changes.
Flush the cooling system and change coolant once a year. A 50/50 mix of coolant and distilled water will keep the cooling system in good shape and prevent corrosion and deposits from building up inside the cooling system.
Change out transmission and differential oils. While not requiring frequent service, these fluids must be changed according to service intervals. Always use transmission fluid or gear oil of the recommended type and viscosity.
Keep it clean. While washing the outside of the vehicle is obvious, most everything the vehicle ran over can also get stuck to the underside. Hosing off winter salt and road grime is a good idea.
Everything with moving parts needs grease to survive. This ball joint went into early retirement due to poor lubrication.