Do people seriously have nipple issues?
Body Glide or some other brand of the same thing is great but make sure that you really scrub the area clean afterwards. It will cake on and cause you other sorts of problems. Good hygeine is essential when spending long amounts of time under shitty conditions. Diseases and infections can run wild. So remember when you're caking all that stuff on to make sure it all gets off.
With a lot of cleaning, it can help save blisters. I used to get bad ones at OCS. Tape the blister up, and cover the outside of the tape with body glide. It will keep your sock or boot from "grabbing" onto the tape and rubbing it up and down over your blister.