I just booked a flight to Slovenia and found a sweet deal on travelcuts for $1000 return. I waited to book until the following day and it had gone up a hundred bucks. After I booked the following day it went up $300. My advice is that if you see something that looks good, book it because it won't last.
When I was searching I mostly used all the standard websites like Travelcuts, expedia, travelocity. There are also some more discount sites like flyzoom.com but the departures didn't match my trip window. One thing to remember is that if you can catch a cheap flight to London, there are any number of discount flights from there. I would look into stuff like skyeurope.com, easyjet.com and ryanair.com from London.
Good luck, I found the whole process to be very frustrating but once it's done you can just focus on your trip.
Edit: you can also use sites like
www.farecompare.com to search for the cheapest flight from a variety of airlines.