This is our experience too. I booked my next trip 4 months in advance and I thought the fares were high but I was needing an island fix. Fares have lowered a bunch since then. Its now two months away and the fares are more inline with the past.

Another key about Dane's statement is whether they have seats left as time passes. A couple of the flights we originally considered don't show up anymore on searches since they're full. This becomes very important if you're flying from a small airport on a commuter flight to a hub since those often book full quickly.

Usual guidance is:
1. Its a crap shoot.
2. Use the internet search engines to watch airfares
3. Watch the fares enough that you know what a good deal is or isn't.
4. Book at any time you think the fare is reasonable and budget acceptable.

Do searches here on this topic - you'll get lots of good advice.