I generally book at least three months in advance. I watch the price each day, and when it starts to creep up, I buy. My home airport is Orlando. International flights from Orlando are almost always significantly more expensive than from major east coast cities. While I always check fares from my home airport, I also keep an eye on fares from major cities.
We go to Istanbul each fall. Fares from Orlando two weeks ago were $1200, but the fare from JFK was $585. A positioning flight from Orlando to JFK was $215. So I was able to obtain the entire trip for $400 less per person than I would have paid flying out of my home airport. Last year I had a similar experience in Chicago. If you are flexible with your dates and cities of departure, you can often find lower fares.
Sometimes the day of the week makes a huge difference. If you use matrix.itasoftware you can see that there is often as much as $300 to $400 difference from one day to another within the same week.
I also try to stay informed about the airline industry. When Turkish Air announces that it is opening a new market, I can almost always expect Delta will match TY's introductory fare. When they opened in NYC, Delta matched. When they opened in Chicago, Delta matched. They are opening in Boston soon, and I will watch Delta. If they match we will fly to Istanbul in the spring.
Once in a great while it is less expensive to purchase two one-way tickets than one round-trip ticket. Usually it is just the opposite, but I always check because there have been a few times when one-way tickets were less expensive.
My rule is that I buy my airline tickets when the price is appropriate for my budget, and I do not look at the fares after I buy them. Well.....I do not look at those fares until about 10 days before we fly when they are always twice as much as they were when I purchased them.