Finger independence
A drill that crosses between two adjacent strings.
Keep planted fingers down. Fingers that don't need to move stay pressed on the string — only the finger that's moving lifts.
Watch the fingers that shouldn't move. Use a mirror or record yourself to catch fingers lifting or curling along with the moving one.
Use the fingertip to mute the next string. Rest the fingertip on the next string to avoid accidentally hitting it when lifting a finger.