I don't use prep (other than I shave in the shower) for my body. I assume it isn't needed because the hair isn't as coarse as facial hair and the skin is tough. I just keep the correct blade angle and go. Never got any sort of irritation or ingrown hairs.
If I used prep it'd probably be lather on armpits. Hot towel, pre-shave oil, etc all just seem like overkill.
So a warm shower is enough to soften the hair on all body parts that i wish to shave. I've also heard that using conditioner is good to soften the hair.