We get about 8½ years useful life from our big domestic appliances - I am not an expert in dishwashers or anything else but...
The inner filters eventually fall apart over time and debris gets down into the drain pump impeller fouling it - on some machines you can clear this out by hand (tweezers) by looking for a single Torx screw right at the bottom of the sump (inside) and removing the cover of the impeller.
Make sure you regularly top up with salt and rinse aid (avoid the multi-tabs).
Any dirt on the crocks can only be dirt from the dishes you put in so it helps a bit to rinse off any big food deposits before loading.
Rinsing is done with the water already in the machine and it will often have broken down bits of food debris in it - Rinse Aid is intended to allow the rinse water to drain off rather than form droplets on the crocks - it breaks down the surface tension I suppose. If droplets remain on the dishes the drying cycle will bake on hard any remaining bits of crud so a good quality rinse aid is essential.
Good luck...
Peccavi
November 2009