I run hundreds of airfares each year for clients going on my small group tours to Europe and South America. It is hard to decipher the algorithms or formulas airlines come up with for setting fares. Very little of it seems to make much sense. But there are a few techniques I use that seem to work.
If You Don't Like the Fare Today, Try Another Day: I was trying to book a multi-city flight from New York to Budapest then return from Split, Croatia to New York. I was looking every day and the flights were coming up at about $1900 R/T. I waited a week and tried again and got the flight I was looking at for $895 R/T. So, if you see a fare that seems too high, wait a while and sooner or later, it might drop.
Don't Go Direct: For instance a R/T flight, non-stop between JFK and LIM (Lima, Peru) departing on November 1 and returning November 8 is $1,075. Stop in MIA on the way down and the price drops to $764 (American Airlines).
Book Separate Flights: DFW (Dallas) to CDG (Paris) is $1264 R/T on American Airlines. But buy a separate R/T flight on AA between DFW and BOS (Boston) for $205 then a separate R/T flight between BOS and CDG for $574 and that will save you $485.
Bigger Can Be Better: Albany, NY to Paris runs about $1225 R/T. Take the 3 hour Amtrak down to New York and then fly direct out of JFK to save $588. Worth it? I think so. Flying out of larger airports with competitive airlines can be worth it.