I've often been asked how players can become more aggressive? If a player can get into the mindset that he is confident about winning the ball, he will immediately become more aggressive. Learning about what you can "get away with" as far as fouls is pivotal at the developmental stages. Utilizing arms, shoulders, and legs effectively when going in for a tackle takes repetition. Visually seeing professional players do it at the highest level plays a huge part in understanding how to be aggressive on defense.