I found I can get lower rates at the off-airport rental offices near the hotels. I take a shuttle to

You should book (not on a nonrefundable) site, and then start to check frequently. I started at $980 for my BI rental and have gone down to ~$440 for my dates (the price is back up in the $900s now).
I found I can get lower rates at the off-airport rental offices near the hotels. I take a shuttle to and from the airport. Another way I have saved is to only rent the car for the days I really need it. For days just hanging out at the pool or beach, there is no sense in paying for a car to sit in the parking lot.
sf - Maui seems to always be a little more expensive for car rentals. I go there frequently for work and even last Nov which was a pretty slow time, I paid $125 for two days. So the $305 isn't too bad. I would book it and keep checking back for discounts or price drops.
cd-montereybob makes a great point about off airport rentals. I was in Maui this Feb for a convention/work. I usually book with either Dollar or Enterprise because they are the cheapest. The best I could get for 6 days was around $900. There is no way my company was going to pay for that. I went to the Enterprise office in Kahului on Hana Highway. It's about a 5-7 min ($10) car ride from the airport. My rental from that office was $323. When I returned my car to that location, the staff there drove me to the airport. They just have a policy that they can't pick people up at the airport.
I'm sure some of the hotels may have good deals, too. I guess depending where you are staying, you'd have to factor in the cost of a shuttle to and from the airport. But Enterprise in Kahului was a great alternative.