I used to have problems with blisters, then I found out about Body Glide here at the DIS, I used it for 3 trips and didn't get one blister! I forgot it on my last trip and was really worried, then I remembered about the deodorant and tried that and I didn't get one blister that trip either. I did apply the deodorant a little thicker than I did the body glide, but wound up with the same effect, so I've been using deordorant ever since for any lot of walking days I have and am so far still blister free.
Honestly, I think the Body Glide works and so does deodorant, but I think the MAIN thing to do to avoid blisters is wear the most broken in comfortable shoes you've got! They may not be cute, but believe me, after a day in the parks you won't give a hoot what they look like as long as you can still walk in them!
A little OT, but I've also put Memory Foam insoles in my shoes last trip and they were GREAT, I went all day (13 miles of walking) and my feet didn't hurt at all. Love those things!