Check comparison sites like Kayak, Skyscanner, Opodo, Expedia to find the best prices and possible routes for your dates. If you can be flexible with dates and airports and are willing to consider indirect options this will open up the best chance of cheaper fares. Also compare with the airlines direct as sometimes they will have good sale fares or Miles & Money discounts. Prices for the same flights can vary wildly, we booked next years flights with lastminute.com saving over £500 on the same flights direct from VA.
When you see a good price BOOK IT do not hang around as they can be gone in minutes.