Flight time is one of the most important things if you want to travel the most profitable, perhaps equally important time of booking a ticket. Of utmost importance is to avoid flying during high travel season, major holidays or events. Demand is greatest then, and prices, so the first rule is to match its lightest time of the year.
If possible, travel in the days of the week with low traffic. For example, flights on Friday evening and Sunday evening are usually the most expensive, and Tuesday, Wednesday and Thursday are cheaper. To some extent this is true of Sunday morning. Also, if snowdrifts Saturday night can save you money, since you charged on tourism rather than business tariff for airfare.
If graphics and physical resources allow you to change the time of flight you can increase your chances to get cheap airfare. Flying with night flights are often more profitable because nobody wants to take off in one morning and landed at 6 am.