Chafing (raw and irritated skin) occurs in areas of increased friction which can be made worse by heat and moisture.  Common areas include the inner thighs or shoulders for runners wearing packs.

Prevention

Lubricants and powders
Various lubricants and powders can be applied prior to racing and at intervals throughout the race to areas that are prone to chafing. Test your strategy and product out before race day.

Consider your clothing
Synthetic materials (rather than cotton) will dry more quickly and decrease likelihood of chafing.

Clean the area
Periodically cleaning chafing prone areas, such as the thighs, with wipes and reapplying lubricant during activity can decrease chafing.

Kinesiotape
Consider applying kinesiotape or similar on the shoulders to help with chafing from shoulder straps if you are prone to chafing in this area. 

Treatment

if you develop chafing you will need to both protect the area and decrease friction. Apply a thin layer lubricating product directly on the area. This will hurt. If it is an area like the nipples that you can then cover with a bandaid or similar this will help. Note: try not to have adhesive directly on the chafed area as this will really hurt when you remove it. This is why band aids or similar with parts that area adhesive and parts that are non stick are effective.