update Timestamp updated whenever the table is updated

PostgreSQL 9.2.3

I would like to have a generic strategy for updating a timestamp field on some tables whenever the table is updated.  Is there a recommended strategy for doing this other than via the SQL UPDATE command?  I think I read something about RULES.  In the below example I have a table called test_table.  I would ideally like to update the field called “updated_timestamp” whenever an update occurs by doing:

UPDATE test_table SET updated_timestamp=now() WHERE id=?

While looking at the concept of RULES how do I say after an update happens update that particular record and not all of the records in test_table.

Example table:

CREATE TABLE test_table

(

  id integer NOT NULL,

  field1 character varying NOT NULL,

  field2 character varying NOT NULL,

  updated_timestamp timestamp with time zone DEFAULT now(),

  created_timestamp timestamp with time zone DEFAULT now()

);

Thanks,

Lance Campbell

Software Architect

Web Services at Public Affairs

217-333-0382