As with all boxing training, shadowboxing should be done very conscientiously with an opponent in mind. If you spar regularly and/or watch fights, that can serve as reference points in which to direct your sparring sessions.

On a more beginner level, you should be more concerned with learning how to deal with an opponent's punches (usually the jab since it's the punch thrown most), working on the right responses (eg. slip, catch, or parry). Move around deliberately, and don't just punch for the sake of punching. From your sparring sessions if you notice a punch that you keep getting caught with, you should practice the defense against it while you shadowbox.

As you get better, you can work on different situations and scenarios that you're likely to encounter in the ring. Watching fights will give you plenty of things to try as you'll see how a pro adapts to the different problems that come his way. Practice all the different moves, defense, feints, counters consistently trying to get it smoother each time. All of this should serve as reference material for you to use in the ring.

If you're just randomly moving around and throwing punches, then you're not actually actually shadowboxing, you're only pretending to look like a boxer. Always do things out necessity.