Although some have said that Tide was acceptable, I've read that any kind of detergent is unsuitable for washing water repellent / breathable material. Apparently detergent leaves behind a residue that coats the DWR, rendering it useless.
My research ultimately led me to try Dri-pak soap flakes, made in the UK and resold through a distributor in the US. Results thus far have been promising. The DWR held up after the wash, and the soap lifted all of the dirt/stains. Dri-pak is a much cheaper alternative to Nikwax Tech Wash -- about 4 times cheaper per unit washed.
So, how do you wash your DWR apparel and have you tried Dripak?