>Health and Nutrition>Bleeding Nipples (male) (warning - picture with bloodied shirt included in post)
For me, anything over about 4 miles and I'm putting Band-Aids on. I keep my chest hair trimmed down so it is less painful to remove them. To aid in removal, I dotn't take them off until just after I shower. The hot water seems to help. I've never had a problem with them coming loose or falling off. I keep extras in my glove box, gym-bag and in the house. I think I'd rather take a kick in the crotch over chaffed nipples. My first 25k trail run I did, I forgot the band-aids. 4 miles in and I had to take my shirt off. Not very pleasant in 38F temps. Band Aids!!! Get the big box of them. Plus, they work better on cut fingers than "nipple guards".
"Spenco 2nd Skin Adhesive Knit" works great for me.
You'll look like a stripper with pasties if you take off your shirt, though.