best way to auto-update a field when row is updated

I have the following table

FIELD_A        |          FIELD_B          |           TSTAMP
--------------------------------------------------------------------------------------------------------
x                              y                          2005-03-10
14:56:47.456431

TSTAMP = not null default now()

What's the best way to always auto-update TSTAMP to it's
default value whenever the row gets updated?
(e.g. update TABLENAME set FIELD_A='zzz' where FIELD_A='x';
should automatically set TSTAMP to now)