I just booked my flight for September last week and I had a doozie of a time!
I used Kayak.com - you cannot book through Kayak, but it pulls flights from multiple sites (including expedia, hotwire, the airlines etc...) and compares them. I agree with the previous poster - I do buy through the airline when I have that option.
Also, if you go to bing.com and type in the dates and airports to/from, it will automatically "farecast" your flight and tell you if rates are trending up or down. I know that from PA down to FL it was trending upward, and it wasn't getting any cheaper.
If you have the option of a few different airports, play around with it. We are flying out of Harrisburg but flying back into Baltimore - ended up being cheaper and we got MUCH better flight times.
Lastly - remember that Magical Express (if you are using it) leaves your hotel 3 hours before your scheduled departure flight - makes a 9 a.m. flight going home look not so great!