We have used a digital pot to control contrast. Since the contrast voltage is usually near 0V, we put a 1K digital pot in series with a 4.7K fixed resistor. That gives a range of roughly 0 - 1 Volts.

We have also used a temperature compensation circuit in some applications. Setting the contrast for best viewing at 10 degrees C sometimes means that it is barely readable at 30 degrees C.

-- Tom