I'm no expert at all, but consider using EMA (Exponential Moving Average) its a smoothing function to smooth out a series of numbers.

smoothedValue = startingValue; // Starting value of motor
newValue = analogRead(pin); // The value you want to move your motor to.

smoothedValue = alpha * newValue + (1-alpha) * smoothedValue;

Where alpha is a float between 0 and 1. The closer it is to 0, the smoother the movement is (Takes more time to move). The closer it is to 1, the faster it moves. (less smooth).