I have a small collection of Aquaphor samples that I obtained at various pre-race expos. I never bothered with them because I could have sworn that this stuff was meant for babies' diaper rashes. But getting past that, I only need lip balm when the humidity is really low, and before I started running, I never knew what chafing was.
First, a chafing occurs when your skin rubs up against something many times, even other parts of the skin (leaving you with two chafed areas). From my experience, I know when I'm chafing just when the area becomes raw. Often, there are streaks of red in the area, and very rarely, a miniscule amount of bleeding. It hurts when sweat, sunscreen, wind, or anything else hits it. And don't get me started on the post-run shower. The chafing improves on its own over time (a phenonmenon I like to call the "chafing callous").
After low mileage for the past two months, I started chafing when I kicked it up again recently. To protect myself, I finally decided to use some Aquaphor on it. The stuff feels like a thicker, less messy version of Vaseline. It is colorless and scentless but quite persistent on the fingers. I must say that this stuff really did work for me, but not in such a way that I felt the areas were slippery. I would not feel too great putting this stuff everywhere, but since I know I only chafe in a few areas, I can just target those.
I wish I had used some during the marathon, as I had slightly raw underarms after. I also made the mistake of stuffing a gel in my sports bra for the first half of the race and found that it skinned me right near the heart. A month later, I realized that the shorts I was wearing had a hidden pocket inside. D'oh! However, I will remember the Aquaphor if I ever decide to use it.
P.S. Men, I know you have a special problem with chafing, which involves a shirt rubbing against the chest while racing. I don't think Aquaphor will work for that, so stick with the band-aids.