To limit the chances of a propane tank freezing up, please follow the below recommendations:
Start with a full tank, the fuller the tank the less chance for a tank freezing to occur.  The larger the tank, the less chance for freezing as well, since you will have more propane in it.
When its cold outside, there is also a bigger chance for tank freezing.  If temperature is 85 degrees or higher, better chance for it NOT to freeze.