We've been renting cars in Hawaii for years, and the best strategy is to book really early. I typically look on Expedia or Orbitz as far out as 11 months if we know what dates we will be there--I realize everyone does not book 11 months out, but the earlier the better. Check rates and make a reservation for exactly the dates/car type you want--there is no fee to make the reservation, and you can cancel without any cost. Then, check the same website you booked with every 2-3 weeks--same dates, same car type--you will be amazed at how often the rates go up and down. If the rate becomes lower, pull a Steve McGarrett and "Book 'em"--you can then cancel the older, higher rate. Repeat every 2-3 weeks. We were in Ko Olina for 11 days over New Years and our rate for a full size car for all 11 days through Alamo was $330--I had booked it probably 9-10 months earlier. If you are booking just 2-4 months out, you may have to pay the prevailing rates, but make a reservation and keep checking for a lower rate--you can always cancel if you find something cheaper.
Again--book AS EARLY AS POSSIBLE for the best rates. (Also--a tip when booking on Expedia: they give you their "preferred vendors" rates first--be sure to check ALL the rental companies to get the best prices.)