Changing default user's passwd

We've been trying to
alter user postgres with passwd '...' createdb createuser;
and the like, and we get
alter user
output showing that the command is successful.

However, when we later psql -U postgres postgres
we are getting:
psql: FATAL 1: IDENT authentication failed for user "postgres".

Is there a way to change this default user's password to avoid su'ing.

Thank you in advance,
r