When I’m planning a trip, I set up flight alerts to let me know when airfare between my airport(s) and the destination airport(s) drop. My go-to site is AirfareWatchdog.
The site lets you search current fares, which is nice, but I still prefer Kayak’s search function. I love AirfareWatchdog’s My Fare Alert function. The alert function let’s you choose your departure and destination airports and they e-mail you when prices drop. There’s an option for “Anywhere that’s cheap” from your departure airport, too. They’ve also added the “Meet me in” option which lets you choose a destination airport and they send you cheap airfare from multiple locations (Might be great for a destination wedding or a reunion).
It’s a free and useful service. It’s taken me to Hawaii a few times.