United Airlines has some good deals from Newark (NYC Area) to Miami for the month of March. Deals start at $89 roundtrip for the restricted fare class (no carry ons, EQMs etc.), but if you pay $40 more you would get all those benefits + more frequent flyer miles.
The best way to search for these deals is to check Google Flights. So far I’ve seen availability for March 14-20, March 6-14, March 17-20.
And it looks like AA is available on some of those dates without restrictions (which means more miles, better seats with a connection though)
Update: Chicago-Miami is also available, it seems, on certain days in the same proximity of dates mentioned earlier. Check Google Flights for more of the deals.
Anyway I know some of you are more eager to hear about less restrictive fares, but I just thought I would let you know about these unusually low fares (normal rates are around $300+). Flying United or American even with restrictions is still often times better than flying Spirit because if you miss your flight or connection there is always another flight afterwards.
I would bank United flights to Singapore Airlines and AA flights to Alaska Airlines or British Airways normally. But please check the earning pages before proceeding because these are likely to be on the low end of the fare class spectrum.