All depends on time of year, Is it school hols? Are you willing to be very flexible?
We have done 1 or 2 last minute but no great savings. Best deals tend to be with Tui, scheduled airlines seem to go up last minute. We also tried to do last minute one Xmas, ended up staying at home, no flights left unless paying a fortune.
Booking in advance and finding a deal is best way for us, this year flying direct to Washington, direct back from Miami, £340pp incl luggage and seat selection, not bad for Virgin flights.