Plane tickets can be the most expensive single item on your travels, and timing the purchase correctly can easily save you hundreds of dollars.
While planning is generally a key to saving money, purchasing tickets the day they go on sale (most airlines will publish fares 330 days out) isn't necessarily the best way to do it. Sometimes, airlines will add more flights to a popular route as the departure date grows, thus adding competition and likely lowering the fare. Some airlines, such as low-cost darling Southwest, don't even publish fares until six months before flights.
However, don't wait too long. Airlines usually have preset price increases based on how far out from departure you are. For example, when your departure is within 90 days, 30 days, and so on, the prices go up based on airline pricing rules. For this reason, the best time to purchase flights is usually six months in advance. I'll start looking seven months and then give it until five months out before pulling the trigger.
Of course, sometimes you can't avoid a short window. If you have to purchase a last-minute flight, that's usually the best time to use your points. While published fare rules may require a certain minimum price within these preset timeframes when using dollars, mileage prices are usually less affected and have more standardized price points.