If you want to improve your doubles game, start paying more attention to how you move when you are at the net. If you feel like you are standing there flat-footed or waiting for the ball to be hit your way, then you are not playing the net the right way. When you watch high-level college or professional doubles, start paying attention to how the net players are moving at the net. You will notice that they are almost constantly in motion, always attempting to position themselves in a spot where they can get the ball. Once you understand how they are shifting and why, you can start implementing this into your own game, and it will be much more of a factor at the net rather than being a spectator.
The simplest way to explain the shifting is that if you are at the net, you should always follow the ball. When the ball moves to the far side of the court (your opponent is hitting a shot from the baseline or behind the baseline), you should follow it up and look to make a move on their next shot. If the ball gets behind you (your partner is hitting a shot from the baseline), you should slide back and be ready to cover the middle. You want to slide back and towards the middle because you want to be in the best position to respond to the next person who has a chance to hit the ball after your partner. If you are playing traditional doubles and both sides are playing one person at the net and one person at baseline, then the next person with a chance to hit after your partner is the net player across from you. If you don’t slide back, then you will not have enough time to react to their volley, and if you don’t slide towards the middle, then you are leaving a big hole in the center of the court. Once the ball gets by the net player on the other side, you want to follow it over (don’t leave the down-the-line shot wide open!) and back up so that you are in an aggressive position to play on your opponent’s groundstroke.
Next time you play doubles, try to focus on this shifting! You should start seeing more balls immediately, but even if you don’t, at least you will feel more involved and active in the points!