Take lots of jump shots over the spring and summer. Take them off of the pass and off the dribble. Make sure you also take several shots once you are tired because you always want to be sure that you can make them even while you're fatigued. In games, many times the reason for missed shots is muscle fatigue in the arms or legs. Prepare your muscles by always working hard when you work on a skill.
Defense also improves with repetition.
A defensive slide is an awkward maneuver because in everyday movement you don't move laterally in that way. That is a good reason to practice defensive sliding in both directions, at top speed. The stronger they are, the more explosive you can be in stepping to caught off your opponent. Defensive slides the width of the key for 30-second intervals are good. Slides in alternating directions, zigzagging the length of the court are also a good way to work on your defense.