Most of us that make multiple trips to the islands already know this, but if you have a planned visit and are going to rent a car, keep checking the rental rates for your trip when you get within a couple of months. Rental car rates and promotions change, as does inventories.
Example: My 13 day rental for July was a constant $515 (mid size) on Costco and DHCR until the past two weeks. I watched it go down and stay around $425 for a week, then 2 days ago I checked and it was $327! I re-booked, checked again yesterday and today and it has moved back to $515 again. It's a game that can save you $$ if you stay on top of it. Last year, my best rate on Maui was booked 3 days before we arrived.