ATTQOTD: Before each run, I use Body Glide on my breast bone where the center of the sports bra will sit and then also just below my armpits and the corresponding areas on my biceps. Depending on the humidity and length of the run, the pit/biceps combination may chafe. If I'm wearing my hydration vest, I know that the edge sitting on my left shoulder will chafe about a 1/2-inch line, so I will Body Glide that. Otherwise, I'm good to go. If my sports bra starts chafing in more areas, then it's a sign that it's getting old and needs to start rotating to shorter runs and then out.
My worst experience with chafing was a half-marathon race in the pouring rain and low 50s F temps. It was cool enough (and I was inexperienced enough) to think that I need to wear long sleeves (and maybe a tank under - don't remember), so there was way too much fabric involved. I knew I was going to get chafed from getting soaked, so I tried to use Body Glide on EVERYTHING before the race, but it just wasn't enough. I ended up with chafing along all fabric edges and seams and wow, that was painful. While I will still run in the rain, I don't think I've ever gotten quite as soaked as I was after that race.