Recipe from Flitterbugs blog.
1 cup borax
1 cup washing soda (she says that baking soda works just fine too, we just already had washing soda)
1/2 cup Kosher salt
1/2 cup citric acid (found with the canning stuff, or use 15 packs of unsweetened lemon Kool-Aid)
Mix all ingredients, except vinegar, in a air tight container. Use about 1 Tbsp per load in the dishwasher. Don't skip this step. Fill rinse-aid container of you dishwasher with white vinegar. This helps to prevent a cloudy build up, especially on plastics.
She recommends using a regular detergent once a month for one load to prevent the plumbing from clogging up. So far it has been working just as good as store bought, plus its much less expensive.