How to avoid (stop) a endless loop in a trigger

Hi people, i have a problem here.
I'm doing a trigger that when a update occurs i need to do an update on the same table (target table), but as known, it causes a endless loop whithin infinit updates. So I need to stop the trigger after it does the first update, is there any way?????
I tried to do a return null, but that was a very bad idea because it stops completly the function fired by a trigger and all its computation is in vain...

The test trigger that i did is like :

CREATE OR REPLACE FUNCTION public.sp_teste_loop() RETURNS trigger AS '
begin
raise notice \'Trigger Fired\';
if (TG_OP = \'INSERT\') then
update teste_trigger
set flg_bool = \'S\'
where codigo=NEW.codigo;

RETURN NEW;

elsif (TG_OP = \'UPDATE\') then
update teste_trigger
set flg_bool = \'N\'
where codigo=NEW.codigo;

RETURN NULL;
end if;
end;
' LANGUAGE 'plpgsql' VOLATILE;

CREATE TRIGGER tr_sp_teste_trigger
BEFORE INSERT OR UPDATE
ON public.teste_trigger
FOR EACH ROW
EXECUTE PROCEDURE public.sp_teste_loop();

Thank for any help and regards!!!!

=====================
Rodrigo Sakai
Database Programmer
rodrigo(at)2bfree(dot)com(dot)br
http://www.2bfree.com.br
Tel: (55) (11) 5083-5577
Fax: (55) (11) 5549-3598
=====================