Don't get fooled into thinking that last-minute deals are necessarily the best option for cheap flight tickets because they seldom are.
The smart move is to book early, even as far as a year in advance if that is possible for the flight you require. Flight tickets have a tendency to be cheaper when they are first released – which is normally about a year before the flight – so have a look online as soon as the flights you want are available.
This is particularly true if you are planning to travel during school holidays or to a destination where a famous event is taking place. These prices will increase rapidly, so book before the masses do.
The price of a flight may drop again before re-climbing in the last few days before the flight takes off but they also may not. It's not worth the risk of waiting for a drop in prices only to be faced with higher prices for leaving it until the last minute. You also run the risk that the tickets will become sold out before you book yours, especially if it's a popular flight.
If you're willing to be flexible, which is really the name of the game for cheap flight tickets, then have a look at a flight search engine as it may post flight tickets which get lower in price as the flight time approaches.