App note from Analog Devices on ways to eliminate drifts on voltage refs. Link here (PDF)
Long-Term Drift (LTD) in voltage references is a parameter that can be confusing and misleading. It is given as a typical parameter in data sheets, but can cause significant accuracy limitations in system. Unlike tempco and initial accuracy, where a one-time calibration can be used to eliminate these errors, attempts to reduce LTD requires frequent system calibration. This can be both time consuming and expensive. Therefore, it is important to understand the significance of LTD.