ALTER SESSION might match your needs for now, but it is a very bad habit IMO to rely on the session date format. I'm pretty sure you will regret it in the long term. So as a short-term solution that can be OK, but keep in mind that this should be changed in the future.

What we do in order to use the same format everywhere we need to is using a date format as a package variable and use it instead of hard-coded string format. For example :

Code:
CREATE OR REPLACE PACKAGE PKG_Dates AS

    GstrDateFormat VARCHAR2(25) := 'YYYY/MM/DD HH24:MI:SS';
    GstrTimestampFormat VARCHAR2(30) := 'YYYY/MM/DD HH24:MI:SS.FF6';

END PKG_Dates;
/
This does not work in pure SQL since package variables cannot be called from there, but works perfectly in all PL/SQL code. For example :

Code:
rbaraer@Ora10g> var rc REFCURSOR
rbaraer@Ora10g> BEGIN
    OPEN :rc FOR
        SELECT TO_CHAR(SYSDATE, PKG_Dates.GstrDateFormat),
            TO_CHAR(SYSTIMESTAMP, PKG_Dates.GstrTimestampFormat)
        FROM DUAL;
END;  2    3    4    5    6
  7  /

PL/SQL procedure successfully completed.

rbaraer@Ora10g> PRINT rc

TO_CHAR(SYSDATE,:B2)
---------------------------------------------------------------------------
TO_CHAR(SYSTIMESTAMP,:B1)
---------------------------------------------------------------------------
2006/08/22 10:49:54
2006/08/22 10:49:54.949515


rbaraer@Ora10g>
HTH & Regards,

rbaraer