The best advice I can give you is keep checking. If you haven't already, sign up for the frequent rental programs and make sure you're signed up to receive their emails for special offers. Mousesavers is a good website to check - they post all the available discount codes, coupons, etc.
See sites like bnm.com, rentalcarmomma.com, vacationoutlet.com (and click on 'car rental' - it'll bring you to a page with discount codes); mousesavers.com goes without saying, of course; hotwire.com over priceline because often you get to see the rental agency before you make the booking.