I got my flights slightly cheaper through a travel agent as they were able to negotiate an agents discount! If I were you I'd check out the web prices then go into a travel agents to see if they can get you a better deal.
We've been to Flordia a few times in recent years (my parents have a holiday home there). I've always got cheapest flights on the internet, normally via opodo. Don't know where you're flying too, but it's worth looking at indirect flights as well as direct as you can save a lot of money - although it can take a lot, lot longer! We flew indirect a couple of times but now we've got children I'd rather pay the extra cash and go direct as it's such a long trip anyway.