December is high season, so that's tough.

I like using Orbitz because you can do a search for a 30-day period to see which days and which airlines will give you the best fares. They also have a price assurance so if someone books the same flight as you at a lower rate, you get a partial refund. You can also set up alerts by entering the dates you want to travel and the price you want to pay and they'll email you if they find a fare for you.

This is how we booked Brian's last trip from Montevideo to the US.