Take a look at prices now. Determine how much you feel comfortable spending. If the price goes below that number, consider booking at that point. If you do that, do NOT look at prices later. They may still fluctuate.
If your travel plans are at all flexible, it sometimes makes a difference whether you're flying on a Monday or a Wednesday. It also can make a difference if you're flying out at 6 am or 2 pm. Search sites like Orbitz, Kayak, Travelocity, etc., allow you to play around with those variables and have an idea of what might be cheaper.
Your departure airport can also make a difference on the cost of your flight. If there are other airports nearby, it's often worth your time to plug those airports into your search engines and see what's the cheapest option.
And last, but not least - always book directly with the airline you wish to fly. In fact, as soon as you book your tickets, sign up for that airline's frequent flyer program. You may want to do this again in the future!
And somebody else will correct me if I'm wrong here, but I don't think Southwest flights show up on most of the search engines. The same may be true for Jet Blue. These are both discount airlines which may offer better fares for the dates/starting destination you wish to fly.
Hope this helps, and have a wonderful trip!