I like to soak in a bath before I shave, which prevents this because the skin is soft. It's also important to you a clean, very sharp razor, and moisturize afterwards with a gentle, hypoallergenic, noncomodogenic lotion (like Cetaphil cream). Also, wear soft, comfortable cotton underwear if you can, because these will be much less irritating to your skin, and any sweat will be absorbed rather than sitting there and causing irritation as happens with synthetic fabrics.