I always use vacationrentals.com and get a condo down there when I visit. Usually cheaper than a hotel down there and comes fully furnished and everything. Have to contact them at least a week in advance to set something up, but you can find some NICE places there. Even in some of the hotel/resorts from year-round suite owners (Setai/Fontainebleu).
South Beach is a LOT of fun. Bring $$$$. It gets spent FAST. Personally, I like to stay in downtown Miami, which is a 5-10 minute drive to SoBe. Usually stay at a condo across from Bayside and down the street from the AAA.
Pubbelly and Burger and Beer Joint on South Beach are some real good eats. There's also a Shake Shack on Lincoln. In Downtown Miami there's Pollo y Jarras and Ceviche 105 right next to eachother. Like Prison Mike said, the Clevelander and Mansion are great. Personally, I like the smaller, more chill places. Purdy Lounge is always a good time.