Bid using priceline. It always works for me. Last time I used them I bid on a small car and got an SUV at the rental agency. There is a trick that works usually in MIA during busy season when using priceline. Always bid on the second smallest size (I forgot the exact name for the size), they never seem to have them at most major MIA car rental agencies and I always get upgraded to whatever I want. The only agency I have never had this work with is Enterprise, they always have the small cars.
Be warned though, this is NOT a guarantee, some agencies will make sure that what you order is what you get, others just take reservations and will give you whatever they have.
Also some agencies have lot attendants who basically just stand there and point you to your car, sometimes if you ask nice they will just upgrade you....however keep in mind that they expect a tip for doing this...$10-20 works great and makes them happy and usually they'll walk you to a nicer car with a bigger tip.