This post is primarily for men, since women wear sports bras and don’t typically have this problem.
Nipple chafing on long runs can be a painful problem and can result in very unsightly bleeding. Specialized products such as NipGuards have been developed to prevent it. NipGuards sell 10 pair for about $9, making them an expensive proposition for long distance runners.
I use small, round spot bandages from my local CVS store, which sell at about $4 for 100 (50 pair), or less than 1/10 the price of NipGuards.
I have heard that some guys use BodyGlide, Vaseline, surgical tape, and even duct tape to protect themselves.
What do you use and why? Tell me in the comments.