We've always used www.kayak.com
for price checks on domestic and international airfares. They scour the airline websites as well as the big agencies (orbitz, vayama, cheaptickets, expedia, travelocity, priceline) and give the best available fares. It's a one stop shop.
One of the best features they have is their price alerts where you can set up your city pairs and time of travel and set your price limit. It will track your flight costs and send you an email daily or weekly (depending your preferences) as well as the price trend of your itinerary. Airfarewatchdog
(two other great sites) also offer similar alerts.
Since Frankfurt is Lufthansa Airlines'
hub, sign up for their email specials or follow them on twitter to get a heads up on possible deals too. Hope that helps!