If you look through some of the columns at smartertravel.com (especially the student section), they have some more generalized recommendations, like when you should buy for thanksgiving or spring break.

In my experience, it has always been a hard question to figure out. Generally I will watch fares for a couple of weeks and then buy something when it fluctuates back to the lowest price I have seen. I also use rules of thumb such as any transcon to PDX around $300 is a good deal.

Finally, the best time to buy tickets from (and perhaps to) Alaska is when the oil checks come out and Alaska has a special sale to capitalize on the whole state going to Hawaii.

I'd be happy to answer additional questions on PM.