Start now and ask for the Marriott's or Embassy. If you can do without a 2 BR your chances are somewhat better though the Embassy and one of the Marriott's is mostly 2 BR non lockoff's.
Debbie,
Dean has provided a tremendous amount of info concerning Hawaii exchanges on a somewhat recent post. I don't know the link but I am sure if you look under his user name you will find tons of info. I think you will find that post very helpful.