I use the recipe found here, but there are many variations on this theme.
Here are the ingredients...
2 bars Fels-Naptha
1 cup Borax
1 cup Arm & Hammer Super Washing Soda (this is different than baking soda...just to be clear!)
My mom and I think we have our system down to a science. We grate the Fels-Naptha with a cheese grater and then she pulverizes it in her VitaMetaVegaMin Mixer. OK, it might just be called a VitaMix or is a VegaMite, anyway... we sort of felt like Lucy grating up laundry soap. Then you just add in a cup of each of the other ingredients and you're good to go. 1 Tbsp for a regular load, 2 Tbsp for heavily soiled loads. I have a high efficiency machine and it works really well.
Any other homemade detergent fans out there? What's your recipe?