When is the best time to buy a flight?

Try FareCast. They use all kinds of metrics to predict whether prices are likely to go up or down on a flight-by-flight basis. (Once you find an itinerary you like, I’d suggest looking for it elsewhere as well, as sometimes you’ll find better deals on the airlines’ own sites.)

ASAP

yeah, the sooner the better. airlines are cutting flights drastically right now to save money by not flying with empty seats (gas prices, blah blah). I booked my thanksgiving flight 3 weeks ago on southwest and about a third of the flights i looked at were already booked. it’s freakin’ august.

also don’t forget to add in checked bag fees if you’ll be checking, when calculating the best price. they differ for each airline; southwest is the only one that’s not charging (yet)

Check Kayak.com. The prices they show include all taxes and fees, so no surprise when you click “buy”.
Also, the day of the week you fly on can make a huge difference—leave on a Tuesday or Wednesday instead of Friday or Saturday.
I’ve found that it does make a difference when you look for fares during the day—Sundays seem to be lower than other days for some reason.
If you are going to smallsville, try flying into New York or Boston or another large city then renting a car or taking the train. It might save you a lot in the long run. Providence, RI is usually less expensive than Boston…go figure.