Focus your early offseason training on fundamentals and skill/strength development….batting tee work and soft toss drills are a great way to improve your swing and correct any flaws you may have. Stress quality over quantity! Don’t rush into hitting off machines at over inflated speeds or worry too much about how far the ball is going. As your technique improves gradually increase the number of repetition and watch those things take care of themselves.
Pitchers should concentrate on throwing first pitching second. Let your arm strength build up and as it does increase the number of throws as well as the distance at which you make them. Long toss is a great way to add velocity as well as develop better throwing technique. Secondary pitches (curveball, change up, etc) are only as good as the fastball that accompanies them so building up arm strength will improve upon your fastball as well as keep your arm injury free.
Remember the method you use and process you go through are far more important than the early results!