Please read all of this to understand how to get the best deal. During busy times you will not find many bargains. They are dependent on open inventory. Do your research through Kayak.com, Carrentalsavers, Priceline.com. Costco. Costco & carrentalsavers puts coupons in automagicly. So know your "Best Pricing".
Summer months, holidays & school break it is hard to find the deals. Making reservations sooner is best. I will even reserve several sizes & dates if I am unsure how many people will be with me or when I am going. You get to cancel without any payments/penalty so make those cheapest reservations..
Get an idea of what pricing is with kayak.com. Costco; I find the best deals here generally. Priceline; proprietary pricing so you may want to check them too. Priceline also offers $11 a day car insurance if you need this. Carrentalsavers; Use the pull down menus for "Location" MCO and FL for the automatic coupons to be added.
Reserve your cheapest price now; don't pre pay. Keep watching prices & hotwire.com. If you find a cheaper price, reserve it and then cancel the old reservation. In that Order. Only bid priceline if you understand the cheapest pricing for needed time.
I found the best pricing 10 to 4 weeks before. Mostly 6 weeks is prime Also it is a good time for Priceline bidding; Beware of bidding. You can end up paying more. Note final price when bidding. If you bid check the web sites on how to price your price bid.
Beware of offsite rentals. It takes time to get your car when you wait for a shuttle. Most are a walk across from baggage. Unless you get a great savings pick the at airport rental; Alamo\Enterprise\National, Avis\Budget, Dollar\Thrifty, Hertz\Firefly & EZ. Most sites will state shuttle or at airport. Use this to help in your decision. Personally I like Alamo, Dollar and Thrifty as you get to pick your car from your car class.
I check the pricing daily. Car companies also make mistakes and put cars in at wrong pricing. If so, they will honor those reservations regardless of how low they are. Just keep swimming/checking.
I mostly go off season so pricing is better and competitive. Again, 10 weeks have been prime for me. Start bidding at 10 weeks. Do not over bid! Not all Priceline & Hotwire cars are at MCO anymore so beware.
Bidding is discouraging. Remember bidding is prepaying so be sure of your rate. Be patient. Bid stupid low starting 10 weeks out. Keep bidding every 24hrs. It may take weeks for your bid to be accepted. If hotwire has price you can generally bid lower. Hotwire can drop 5 weeks before needed time.
Most of all know your pricing. I have rented a full size car for a week for $45 total with all taxes included at MCO. I have also paid as much as $150. So know what is your pricing for your time. Good luck and happy hunting.