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
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