From my observations, about two months out the prices rise again, although I think most airlines have a complex structure based on passenger loads, and the prices might dip again if the flight is empty. In July I was on a flight and the person sitting next to me had booked the week before and only paid £70 more, and the flight was pretty busy. I'm looking at flights to SFO for the end of January and NWA had a deal for £280 return (stopping over in MSP), last week it jumped to £331.