logging table changes

hi,

how can i "easily" log changes to individual fields in a table without
naming these fields explicitly? i looked at the auditing example in
http://www.postgresql.org/docs/8.3/static/plpgsql-trigger.html, which
looks promising, but i can't think of a way to "easily" query the table
when i want to know when a certain field changed. i was thinking of
adding a field in the audit table which contains the names of all the
fields which changed, but i don't know who to "easily" get these names.
for some reason i think that there might already exist well known
solutions for my problem so i dare to ask here.

thx for advice

matthias