How to find cheap flights?

[OREOSpeedwagon]

I've been planning a trip to England for spring break this year and Expedia is telling me the cheapest round-trip flight to London I can get is ~$575 (I would like to spend around $900 total). I remember in some thread a while back a guy said he could could go to Europe for a week and his total cost would be around $800. He said you just have to know where to look to get cheap flights and I'm wondering how (or if) I could find one for around $450 or so. Also, any tips on traveling to Europe on a budget would be greatly appreciated