I tend to start looking as soon as I know the dates I want to be there. Having used the same airline for several years now (jetblue) I know what the average price should be and will pull the trigger when I get into that average range. After I book I keep an eye on the price and if I see any reduction I call up and they credit me the difference. This probably changes airline to airline but that is at least how I personally handle it. Check your personal airline of preference and see what their policy is.