To avoid hard water build-up, you need to put salt (regularly) in your dishwasher (found next to dishwasher tablets in stores)... there is a space in the bottom of the machine for it. You can use vinegar instead of dishwasher-rinsing fluid as well (glasses don't turn blue like with dishwasher rinse).

Also, once a month run the dishwasher empty at the highest temperature.