Check airfare prices from three nearest airports. Most sites have an option to let you check nearby airports - different airport combinations can yield very different prices.
It's a good idea to check airfares often because prices fluctuate all the time.
Why do I need to check so many websites?
The answer is that there is no single best website every time due to the sheer number of variables involved: specific airlines, availability, specials, times of day, time of day data is fed to the site, booking date, travel date, time of day, arrival airport, departure airport, connecting flight availability, etc.
What's the best time of day to book a flight?
Mornings are sometimes better. Airlines will vary their prices according to demand and also in response to what their competitors are doing.
What are the best days of the week to check flights?
Saturday and Wednesday mornings are often good times to check airfare prices. The airlines tend to release new fares around these times. Saturday mornings are also good times to find "blooper prices" which are errors by the airlines which sometimes result in really cheap airfare. Don't get too excited, though - it doesn't happen very often. Also, since booking is often busier right after payday, it might be better to book after the 7th of the month.