The lesson is know what a fair price is for your flight, then check a handful of sites often until it comes up for around that price. Try to book at least a few weeks in advance.

I can't stress the first point enough. When you buy anything, or if you need to negotiate a price, do your research, and know what it should cost. It's especially true when booking flights, because you don't want to see a price, think it might go lower, and miss a great deal. If you know the fair value, you can recognize a good offer when you see it, and book instantly.