Altering live databases

I am starting to do a test run of maintaining a live database,
and I'm trying to figure out how to add a relation to an existing
table. Lets say I do:

CREATE TABLE something
(
id serial ,
);

And later I decide I *really* wish I had done:

CREATE TABLE something
(
id serial ,
foo_id int4 not null,

foreign key(foo_id) references foo(id) on update CASCADE,
);

How can I update a live database to add the missing relation?

So far I know that I need to:

ALTER TABLE issue_transaction ADD COLUMN foo_id int4 not null;

But how do I declare the foreign key and how do I declare the
ON UPDATE? Is this something that is better done from a
backup? I've noticed the trigger syntax in a backup isn't
exactly friendly, and the TOC Entry IDs are numbered, but I
woudn't know what number to use to create a new one.

Advice?

Greg