What is the best way to replace the ball and spring on ratchet wrenches and breaker bars?

Drill a small hole thru the other side of the knuckle where the ball is, and drive it out with a small punch. Replace the spring, then pop the ball back in by gently clamping it into place with a vise. There's usually enough "give" in the steel to let you force it in and it will retain the ball after.