Avoiding over-training and over-exercising is a great way to reduce cortisol and raise testosterone. Many athletes and bodybuilders make the mistake of working out excessively. Our body really has a weird mechanism. Some exercise does it good, but too much has the opposite effect. Exercising too much causes stress and raises cortisol, which reduces testosterone.
So if you train too much, you end up losing muscle instead of gaining it! Isn't this very interesting?