Create table if not exists ... how ??

I can't figure out the correct syntax...

I have this, but it just keeps complaining about the IF

IF NOT EXISTS (SELECT table_name FROM information_schema.tables where
table_name = 'post_codes')

THEN

CREATE TABLE post_codes
(
area character varying(10) NOT NULL,
district character varying(10) NOT NULL,
sector character varying(10) NOT NULL,
CONSTRAINT post_codes_pkey PRIMARY KEY (area, district, sector)
)
WITH (
OIDS=FALSE
);
ALTER TABLE post_codes OWNER TO postgres;

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - -

ERROR: syntax error at or near "IF"
LINE 1: IF NOT EXISTS (SELECT table_name FROM information_schema.tab...
^

********** Error **********

ERROR: syntax error at or near "IF"
SQL state: 42601
Character: 1

How should this be written ?

Thanks, Jen