Updating a field on one object with a value from another object?

Hi,

I have a field on the Account object (its an additional address field) that I also want to have reflected in the Contact object (similar to the way Account PrimaryBillTo address details does)

but since I can't modify or add Primary BillTo details I am creating  new separate field.

On the Contact object I can set the default value to pull in from the corresponding field in the account object, however, whats the best way to handle it if the value in the account field changes?

How would the child object know when the account field value changed?

Any help would be appreciated.