Seated russian twists! You can do these with your feet on the ground, or the more advanced version, with your feet up, squeezing your abs to keep your body stable. This move is all core, so you should be using your abs to rotate from side to side. Here are some things to watch out for.
Remember to sit as tall as possible, and don't let your shoulders or back round. You've probably figured out by now that unless you're doing a cat/cow stretch, rounding of the spine, back, and neck is bad most of the time when exercising or even just standing still! Same for this - you don't need to round your spine to twist from side to side; you shouldn't be using your shoulders to propel your body but rather your core!
KNEES DON'T SWAY
Remember not to let your knees sway from side to side. They should remain in the same place throughout the exercise.
LEAVE THE "DROP IT LIKE IT'S HOT" FOR THE DANCE FLOOR
I see a lot of people twist and bend until the weight or ball is dropped to the floor. Don't do this. As you rotate from side to side, imagine your weight or ball is circling from side to side around your torso.
Seems pretty simple, right? Remember to engage your core and roll your shoulders down/relaxed as you twist!