The New York Times had a great article this weekend, about why you're probably using too much detergent.
Dishwasher technology has changed a lot in the last twenty years, but people still adhere rigorously to the Eleventh Commandment: Thou shalt pre-rinse thy dishes. Vigorously. With any dishwasher made in the last twenty years, this is completely unnecessary, and in fact harmful to your dishes. Dishwashers are meant to scrub off food--if there isn't any food to scrub off, that scrubbing action gets turned on your glassware instead. This is why your glassware is coming out all scratched. Also, P.S.--if you wash off all the food before you put something in the dishwasher, then why do you have a dishwasher? It's completely superfluous. There's no need to waste the water, time and soap to wash everything twice.
This Christmas, I watched my hubby's family put all their dirty dishes in a sink full of hot soapy water, scrub them clean, wash them off, then put them in the dishwasher. Complete with an overflowing cap of dishwasher detergent. Then at home, I watched my hubby scrub all his dirty dishes completely clean, "load" the dishwasher with a maximum of about nine things, pour at least a pint of dishwasher detergent in, and walk merrily away. Then he wondered why all those dishes came out with a white film of dishwasher soap. When I pointed out to him that both pre-rinsing and a pint of detergent were unnecessary, he looked at me as though I'd turned into a giant cockroach and then insinuated that somehow I'd been raised wrong.
So, to recap the Times article, here are the New Commandments of Your Dishwasher:
1. Do not pre-rinse. Anything. Ever. Leave the food on there. Really. I'm serious.
2. Pack it tight. If you can fit one more thing in there, go for it. Dishwashwers are like freezers--they're most efficient when they're full. (That means FULL. Anything else is just wasting water.)
3. You need about two tablespoons of detergent, MAXIMUM. Most of the time you can get away with one tablespoon. Pay no attention to the little fill space on the inside of the dishwasher door. That is not a measurement. If you must use it as such, fill that up halfway. Most people fill that space to overflowing, and sometimes add more detergent on top of that, usually in the Jet-Dry space. But that doesn't get your dishes cleaner. It just a) wastes detergent, and b) leaves a soap scum on your glassware from too much detergent.
If you don't believe me, or the Times, go read your dishwasher's instruction manual. Yes, that instruction manual, the one you tossed in a drawer as soon as it was installed and never looked at again. It will tell you all the same things.