Best deals are priceline "express deals" in my experience. Even the regular prices are prerty good deals. I usually book a regular deal in advance since you dont gotta pay until u pick it up, just to have a reservation guaranteed. Then i look the day of or day before i need it at the express deals to see if they're cheaper, which they sometimes are. You gotta pay in full when u book express deals and theres no refunds. I once caught a glitch and got a benz for like $30 a day. Just keep checking it cause the prices change everyday, i always get something nice on there for cheap whenever I travel.
Edit: also try to get a pick up location thats not at an airport, the taxes and fees are ridculous at airports. But even with those fees ive gotten some great deals through priceline, cheaper than i could find anywhere else.