Texas A&M recently crunched the numbers and found weekends are actually the best time to book airline tickets because airlines are more likely to lower their fares on Saturday and Sunday to attract leisure travelers.

Studies also show that flying on Tuesdays is the least expensive day to fly;  Friday and Sunday the most expensive.

The least expensive time of day to fly are red-eye flights or any flights that coincide with lunchtime or dinner.

It pays to buy tickets early.  Domestic tickets will be at their lowest price 79 days or seven weeks before your departure.  After that ticket prices tend to climb up slowly, then spike to their highest a few days before departure.  The exception is when airlines have a big sale.

My favorites are:

Kayak – searches numerous flights in one place.
Google’s Flight Search
– lets you filter flights by price, length and date.
Matrix
more search options than Google.
Bing
Travel has a price predictor that will tell you whether fares are dropping or holding steady.
AirfareWatchdog pulls from other websites that don’t show up in regular searches.