This works with the higher quality belts, but could damage some of the lower types. Use at your own risk, but it really helped with my Shurido Silk Black Belt.

Loop the belt around an immovable object, such as a really heavy table leg, or a handrail in cement. You should hold onto the two ends. Start twisting. Twist the belt up tight as far as your strength will allow you. Untwist and twist it the other way.

This will loosen all of the fibers and any of the chemical sizing that is still in the material.

Added by another user:

My first inclination was to say “Train more”.