I wouldn't jump right into using a lacrosse ball due to its firmness. Start off with a tennis ball and see how that feels first.
Other than that, how is your daily posture? Do you sit at a desk all day hunched over? If you are hunched over all day then you might have poor thoracic mobility, tight pecs, and lack of internal rotation.
Work on improving shoulder stability and always do more "rowing" than "pressing" to improve anterior to posterior muscular imbalances.