Think about it! Most of the time you will not have possession of the ball. Half of the time you will be on defense. Of the remaining half when your team has the ball, 80% of that time you personally will not have the ball, as one of your teammates will have it.
So what do you do when you don't have the ball? Do you simply stand still and wait for someone to pass to you? That's not what wins games. There are things to do when you don't have the ball.
All good offenses have good spacing and movement. Except when screening or cutting around a teammate, players should space out about 12 to 15 feet apart. Poor spacing results in bad passes, turnovers, and poor opportunites for scoring. You must maintain good spacing from the ball and fill the open spots of the court.
Perimeter players should space out above the 3-point arc. If you are inside the arc for no particular reason, you are most likely "clogging" things up for your teammates, and you are making it easier for the defense to defend you. By spacing out, they have to cover more territory and will have a harder time with giving help and then recovering back to you.
Like in the "Read and React" offense, you must learn to "read" what the player with the ball is doing, and then "react" accordingly. In any "motion-fill" offense, you must fill the open spots on the 3-point arc.
When the person with the ball dribble-drives or passes and cuts through, other perimeter players must rotate and fill the open spots according to the rules of the offense that your coach is using... you can't just keep standing in the same spot!
Let's take dribble-penetration... the person with the ball drives to the hoop. Perimeter players should "space-out" to open spots on the perimeter for a kick-out pass. For example, when the point guard drives, a wing player might slide down to the corner.
Or on a dribble-drive into the middle, the opposite weakside players should spot up on the arc at the wing or corner areas for a kick-over pass. On a baseline drive, the opposite wing should slide down to the corner for a possible pass across. Be savvy and cut into an open spot on the floor so the passer can find you with an open passing lane.
Post players must react here too... on a baseline drive, the low post player should "I-cut" up to the elbow to make spacing for the drive and to be in good position for a pass from the dribbler. On a drive to middle, from the top, the post player usually should try to space out to the short corner area, or the opposite block.
You must also learn to read your defender and react accordingly. You're on the perimeter and are being denied and can't get open for a pass... you'll never get open standing still.
If you are over-played (denied) by the defender, back-cut through and fill another spot on the perimeter. Or make a V-cut, taking the defender inside and quickly cutting back out for the pass. Or curl around a post player inside. Or set a screen.
Screen for a teammate. After screening, seal the defender and cut, roll, flare, etc. When the defense switches the screen, it's often the screener that gets open. You could lateral screen or down-screen (perimeter player), or you can ball-screen and roll inside or pop back out.
Post players can ball-screen and run the pick and roll (or a pick and pop). Or they can back-screen, or lateral screen for the opposite post player (if playing 3-out, 2-in).
If you are on the perimeter and the ball is being dribbled at you, back-cut through to the hoop and then fill outside. Or your coach might want to you to run a simple weave-screen, or dribble hand-off to get you the ball.
If you just passed the ball to a teammate, don't stand still... cut! Pass and cut and fill another spot on the perimeter. Too often I see a guard pass and then cut aimlessly through, not even seeing the ball, as though not really expecting to get the ball back. But "it's what coach wants me to do".
Cut with a purpose... make a good fake, a hard cut, have your hands ready and expect to get the ball right back for a quick score. When you cut through, if you don't get the ball back, maybe "chip" (screen) a post defender inside to free up your post player, and then get out to the perimeter.
Always be aware and know where the ball is. Be alert to help a teammate if he/she stops the dribble or is in trouble. Be ready to jump in after loose balls. Be ready to rebound when the shot goes up. Be alert and position yourself for a rebound (see the Rebounding Tips). You'll get extra shots and points this way.
Remember, often it is not the person initially with the ball who scores. Instead a good pass to you cutting, or coming off a screen, is where the score comes. So work hard on offense when you don't actually have the ball...never loaf!