Why should you trust me on chafing? Well, I once ran the equivalent of 67 marathons in 67 days with a 25-pound backpack on. This is my toolkit for anyone trying to avoid pain.
Lube up: Anti-chafing products like petroleum jelly or specialized balms are your best anti-chafe defense. Even if your carefully selected fabrics let you down, generous coatings of lube can save the day. You can’t afford to be shy. Particularly downstairs. You want to make sure you’re getting into the places even you rarely go.
Go seamless: I choose shorts, leggings, base layer shorts, tees and running underwear that are either seamless or have flat–lock seams. I always check the seams of the built-in twin shorts and I rarely trust the inner-briefs built into regular shorts. If in doubt, layer up with your own trusted undies.
Tighten up: I choose tighter, body hugging layers over loose fit gear. It sounds counterintuitive but fabric that stays in one place, close to the skin works better than loose-hanging fabrics that move around more.
Avoid cotton: Cotton is heavy and can be abrasive even when it’s dry. When it gets wet with salty sweat, it’s a disaster for your soft bits. Those layers that touch your skin should be made from synthetic or wool-based fabrics. These dry faster and reduce friction.
Powder your toes: Blisters are essentially chafing gone nuclear and they can be harder to avoid. Powdering my feet and socks can help reduce the moisture that increases the risk of rubbing and hotspots.
More/Source: https://www.wired.com/gallery/best-running-underwear/