Buying plane tickets at their lowest price is all about timing. You want to try to buy within 2 to 1 week before the actual flight.
Being from the Westcoast, I typically fly America West because they offer the lowest fares and have the most variety of flight times.
http://www.seatguru.com/
This site is a must to find more information on the airline and type of plane you will be flying in. Seriously check this site before purchasing a seat. If you have to spend $300, make sure its an airline that offers inflight entertainment and comfortable seats
For example I recently flew on Northwest Boeing 757 (300) and on the way back 757 (200). Northwest has got to be the most bare-bones service out there. No inflight entertainment, the seats on these plane are super packed, they don't even offer peanuts with their drinks. I paid $333 for those tickets too. They deserve to be bankrupt.
In contrast, I flew across country on a Delta/Song 757 for $198, the seats are leather, plenty of leg room, and each one comes with its own Direct Satellite TV.
BTW Boeing is coming out with its new 787 DreamLiner. It has alot of new feautures,for instance, the cabin humidity will be increased from 5% to 15%. Simulated day and night time. I can't wait to fly on this thing.