Chafing occurs most often when you sweat and your skin rubs against fabric, causing a red, burning patch of skin. The skin will be raw and painful, and can sometimes start to bleed. (See Reference 1).
Lubricant or Talcum Powder
Fit Sugar recommends using a lubricant, like Vaseline, on your skin, under the knee brace. This will make the skin slippery and prevent the knee brace from painfully rubbing. Another option is to try talcum powder on your knee. This will dry the skin out, and can help to prevent chafing.
Prepare Your Brace
Prepare your knee brace to prevent chafing as much as possible. Cut off any tags or extra pieces that may rub against your skin. Buy a brace that's seamless, as seams often rub against the skin and cause irritation. Additionally, look for a brace that is made of a material that can wick away moisture.
Take Care of Chafed Skin
If your skin does get chafed, be sure to treat it well so that it can heal, thereby preventing further irritation. After exercising, or removing your brace, be sure to clean irritated skin with soap and cold water. Make sure it is clean from all dirt. Then dry the area completely. You may choose to leave the skin uncovered, or, alternatively, cover the skin with a gauze pad to make sure it stays clean. You may also apply diaper rash cream to help the skin heal.