It really depends on dates, etc. If you're too far out, then the airlines have not released their sale tickets yet. If you're too close to the trip, then the sale tickets have all been bought. I started watching airfare in Jan for our May trip. I was concerned because it starts over Memorial Day weekend. Prices were really high in Jan, but by late Feb, the airlines started having sale tickets - so original prices were about $280/person, and acutal price I paid was about $160/person.
I ended up booking on Southwest, because once I could get the tickets for $180/person, I bought them, and then when there was a better sale, I exchanged them for the lower cost tickets. I now have a credit on Southwest, which I know I'll use in the future (only good for 12 months).
So, if you have time, watch airfares and wait for some sales. If you watch a couple times a week, you should be able to catch a sale (normally released on Wednesdays, I think, so at least check every Wednesday).