I had done quite a bit of research in the spring, and here is what I have found:
Historically, prices are high in order to see how many will bite. Why lower airfare if they don't need to. Then, about 3-4 months before the flight, the airlines would drop the fares in order to fill the plane to an acceptable level. Once they reached that level, the fares would soar upward in order to take advantage of last minute travellers since the plane was full enough to fly anyway.
Considering this, I waited until late March to start looking for our July trip. I ended up booking in mid april at $250, which was much higher than I had ever paid, but I felt lucky to get that rate. So, I started searching earlier for our December trip. I booked in May when the fare was around $200. Right now, it is about $380 and keeps going up every week. It appears that the fares are lower right away, then they start creeping up. Sometimes there are some really good last minute deals, but I wouldn't count on it. I would book as far in advance as possible, as soon as you see a decent rate, book it.
FYI, airfare was $200 two weeks ago for our honey moon dates next June. Since we are taking a cruise, we were kind of restricted on which flights we could take. Well, the price was as low as I have seen for awhile, so I booked it right away. IT might end up going down a few dollars, but not having to worry about it anymore (especially with all the other wedding stuff) was well worth it to me.
Hope this helps!