One additional note: While airfares do get progressively more expensive (on average) as you get closer to your flight, if you book too far out you will actually pay MORE. A lot of discount fares don't open until around six months before a flight (approximately), sometimes less. So if you book too far in advanced you'll pay close to a "full fare" (or at least miss out on some of the better discount classes). This, I suspect, is because the airline hasn't yet calculated how many discount classes they'll be opening up, so they just give you a "default"/safe price that they likely won't regret.

Just something to keep in mind from someone who often plans travel 6+ months in advanced. Booking flights 3-4 months in advance is fairly safe, longer or shorter and you might suffer for it.