For some time now I've been experimenting with making my own dishwasher powder. I've tinkered with the basic formula, even at one point adding Kool-Aid to it (for the citric acid--it turned out to be a crucial step in figuring the formula out). In further experimentation I've discovered it makes a decent no-scratch scouring powder as well, more like Bon Ami than Comet.
What I've discovered is that it doesn't work as well as some of the commercial detergents; however I have found that it works just as well as the major " green alternative" dishwasher detergents, if not a little better, and at a fraction of the cost. If you don't like scraping your dishes and rinsing them off, you won't like homemade powder; I found you had to be diligent about getting all food particles off your dishes because the dishwasher powder will not do it for you. Of course, many dishwashers require you to do this anyway, but oftentimes people are lazy. Like, uh, me. But this is so much easier on the environment and the pocketbook it's worth taking the time to properly prepare the dishes I've found.
Here, after much experimenting, is the formula for dishwashing powder...