Be patient and keep watching. Airfare varies considerably day to day over a week. A fare that is 250 today could be 200 tomorrow, 350 on the weekend and back to 250 again on Monday. The airlines continuously change the pricing of flights based on computer algorithms that seek to maximize both the passenger load and the average seat price. The variability of ticket prices on any individual flight is amazing.
I use Kayak, and "pin" the flights I'm most interested in. I also check the fare pricing indicator on Bing, as someone else mentioned. One that I just started to use is Matrix Airfare Search. I like using the time bar feature. I haven't found anything that gives you a better way to compare available flights than this. Play around with it- you'll be more and more impressed the longer that you do.
Once I find a fare that I like, I usually buy it directly from the airline, as this tends to minimize problems if a flight is delayed or canceled.