Book now, but use site like Travelocity that gives a low price guarantee!

http://svc.travelocity.com/info/info_popup/0,2766,TRAVELOCITY:EN|GUARANTEE_TERMS,00.html

Terms and conditions:
WHAT IS THE TRAVELOCITY LOW PRICE GUARANTEE?

1. The Travelocity Low Price Guarantee ("Low Price Guarantee") is available to travelers who have booked travel on Travelocity.com ("Travelocity") on or after August 25, 2006.
2. If you find a Qualifying Lower Rate within twenty four (24) hours of your booking, we will provide you with the following per booking:
* One $50 Promo Code for a future "Good Buy" Hotel or Flight + Hotel vacation package booking on Travelocity and
* A refund of the difference between the price you paid through Travelocity and the Qualifying Lower Rate.

A Qualifying Lower Rate is a lower rate found on Travelocity or another U.S.-based Web site that satisfies the requirements of these Terms and Conditions, as determined by Travelocity in its sole discretion.
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I can tell you from experience of working in the airline industry for 10 years that start looking now for the best prices. I would recommend a site called Skyscanner. The reason is they can scan from any airport to any airport show you best prices on different days etc etc.
Source(s):
http://www.skyscanner.net

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check http://www.farecast.com/ the URL is self-explanatory :)
not sure if they have data for overseas flights tho, but it's worth trying!
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I would buy as soon as possible if I had the trip sure.

Try different options and take note of the prices during a couple of weeks, after buy the ticket.
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