You should book as far ahead as you can afford, considering the cost of trips increase dramatically closer to the scheduled departure date. The reason for this is that airlines want to ensure their planes are filled and airlines need a certain number of seats sold in order to fly. The best deals come down to price and if an airline was to cut prices the closer to the flight that you booked, they would lose a lot of money. For example, if you are planning to visit  Branson Missouri, then you should book at least three weeks in advance. You could pay as little as $99 round trip from LAX. The same route if you wait until two weeks out will cost $400 round trip.