To some degree you can't prevent this 100%. I only have this happen a few times per season. Definitely make sure the lines are lubricated liberally. If the unit gets wet enough and then re-freezes there is nothing you can do other than to get the bike defrost in a warm plae and wait for all the moisture to evaporate. I've had this happen mostly when riding through some seriously wet thick sloppy snow. The rear derailler gets so incrusted with wet snow that nearly nothing can prevent some moisture from getting in the lines. Like I stated however with proper maintenance this is a fairly rare occurrance.
Happy riding,
André