I have found the second skin covered with Leukotape P the work the best for allowing existing blisters to heal.
For blister prevention I usually tape my heals and toes, where I usually blister, with Leukotape P which usually holds for a couple days of hiking. Duct tape never stays put very long with my sweaty feet.
I have also started using Engo blister pads which are teflon pads that you stick in your footwear rather than on your foot. These have been great and usually survive several weeks of hiking.
http://goengo.com/If you are cheap I know a number of hikers who use bubble wrap and duct tape over the areas where they blister which seems to work well at reducing the friction.