A good way to brace for [low rep] squats is to make sure you take a huge breath, tighten your whole core, make sure your core is [neutral]... that much bracing takes a lot of energy. [For high rep sets] all you want to do is make sure you breathe in, tighten your abdominal muscles to some extent at least and stay nice and upright, focusing on keeping your core tight to make sure it's not wobbly...