How to deliver more punishing blows to running backs, wide receivers, and quarterbacks.
Work on being more explosive. Perform exercises like the power clean, clean and jerk and hang clean. Also plyometrics should be a mainstay in training program. Being more explosive will allow you to deliver more punishing tackles.
Work on strengthening your core as well. A strong powerful core will allow you deliver harder hits because the core is the center point of your body. Think of the core as your body's main power generator. All your power comes from the core.
Improve your running speed. If you can run faster that means your body mass is travelling at a faster velocity meaning when you make contact with another player, you are going to hit them at a higher speed and more force.
Stay low when you are making contact. And drive through your opponent low and hard. You've probably heard the old adage in football "the lower man wins." This is especially true and if you stay low and drive hard with your whole body, you will run through your opponent.
Strengthen your lower body and legs. This can be done by performing compound lifts the squat, leg presses, hamstring curls, and deadlifts. These lifts will add strength and power to your lower extremities. You must have a powerful lower body if you want to hit harder in the game of football.
Lastly, be a smarter football player when it comes to tackling opponents. If you are going against a much larger opponent who is running the ball, make him feel pain, by going really low and cutting his legs out. This may cause him to flip in the air or make him think twice about running at you. If you are going against a smaller player or someone of similiar size, focus on their waste, run your feet, square up, get low and unleash a powerful hit that will make them not want to touch the ball.