I use a variety of tools to check for low prices, including but not limited to, kayak.com, farecompare.com, priceline bid checking forums, etc etc.
Also, on Travelocity you can do a flex search (if your dates are flexible by even a few days) to see on a grid what date combos are cheapest on what airlines, then go to the airline's website and mess with things there.
(click on "more search options" bottom left of this screen capture, then enter the range of months you want to fly out on. You can click on a day and it will look in the surrounding four days for the cheapest flight. You may not get exact dates, but atleast you will know when the cheap days for that airline might be)
If you are doing a multi city stop or open jaw, that can jack up a price quickly. Look for cheap stopover or on a flight in and out of Stockholm and fly another airline (hopefully cheaper?) to Vienna. I think United does a free stopover and I got a $11 6 day stop over on a flight to Bangkok via Tokyo on Northwest recently, sort of out of luck. Sometimes you have to call their 1-800 number and talk to an agent to get a cheap or free stopover with them, though.
As a final resort, try searching out of airports nearby. I think kayak will let you do this, try Oakland or LA? You might find better prices, and still be able to make it there on SouthWest or whatever other cheap carriers are around your area these days.