On previous cars (all petrol), I've always done a "big" service once a year;
* Engine oil + filter
* Gearbox oil
* Spark plugs
* Air filter
* Fuel filter
* Brake fluid
For a relatively small amount of money (<£200) it keeps the car running sweet, and gives peace of mind. I plan to have the 500 serviced shortly (minus the spark plugs as it's a diesel). Is there anything else worth doing on the diesel engines that I've missed?
Pollen filter if you have A/C.