I do comparisons online because sometimes it can be cheaper to actually buy airline tickets directly from the airline. Try Jet Blue, Virgin and SouthWest, they seem to have the cheapest flights, but they don't fly everywhere. Generally, if I have the time to keep checking around, I find the best prices. You can also Google, 'cheapest flights to "Miami" or wherever you're going.
Last time I traveled i found out that if you are a CostCo member, they offer discounted rental cars. It was substantial too. That may be true for Sam's club too. I usually do comparisons online because they are all so competitive, but usually Enterprise is the cheapest here. But Budget & Alamo can have good prices too. AAA and Aarp offer discounts. You can also Google, 'cheapest car rental prices to 'Miami,' as well. Cruising online in that way has given me some great prices for traveling.
Good luck!