You have to wrap the ribs really tight in the alum foil. Don't have any air gaps that allow the ribs to steam. Don't add any additional liquid prior to wrapping.
Dead on advice Kim. I foil my pork butts really tight and keep them dry. I put a light coating of BBQ sauce on my ribs before foiling them really tight as well. They are never mushy, and are perfect. It took me quite a few cooks to figure this out, wish I had read Kim's advice a couple years ago.