The first step to booking any flight especially to Europe is check prices online and check them often. It is easy to do a search online for any flights thanks to travel websites like travelocity and priceline, but the check is your search results. You may search on Monday and find a flight for about 500 dollars, but if you do book then or you may get screwed. Simply just pushing off booking at that low price by a day may mean you can no longer get that price hours later. For example, I found a flight into Heathrow London Airport for 450 dollars on Monday, but went back Tuesday (less than 12 hours later) and the price has risen to 600 +. 600 + is still cheap, but the point is to try to get the best deal. If you find a lowfare instantly take it, but never settle for the first quotes you get, check different travel sites and again check them often.
Next beware of special offers, although they look clean cut and awesome, you need to read the fine print. Certain websites such as travelocity, hotwire, and orbitz offer deals to European destinations during off weeks of the summer season. Such offers often advertise on the home screen of the website and are in bold print, and often read something like "Special Offer: Go to London this Spring for $289 dollars out of Boston or JFK", but again never take any thing at face value. Click the link and read the terms and conditions. Fine print is crucial! The original ticket may only $289, but this does not include taxes and fees! Also, you need to pay attention to the dates the offer applies to. The offer may only available until a certain booking date and only be avalible through date to date. For example: Fly to London for $289 dollars from April 2nd-May 29th.
Always incorporate taxes and fees! Your original ticket price is never the final cost. You are going to get hit with surcharges and other fees. You need to estimate about 100-200 extra dollars in taxes and fees. A lot of airlines now have what is called "September 11th tax" that goes towards extra security and other precautions.
Consider booking a flight through an airline's website or a travel agent. Sometimes getting a cheaper flight to Europe is about going cutting out the middle man so to speak. Contact your favorite travel agent and have them do the investigative work, or even go directly to your favorite airline's site to check out their deals. I ended up booking my flight through the Virgin-Atlantic Airlines actual site and that came out cheaper overall then any other deal I could find online or even through my aunt's travel agent.
If you do book through a travel agent or even online, you need to specify which airports you are looking to go through. I found that Bradley International is about $200 more expensive for a European than Boston or JFK. Even Boston was more expensive than JFK, but you also can look at Providence or Newark, all are within a 2-3 drive from here.
Also, everyone loves direct flights to avoid the lay over hassle, but I've found when booking European flights, connecting flights are USUALLY cheaper. Changing planes in Ireland or Spain is often common and cheaper when flying into London.
Overall, in order to get the best flight to London or any European destination, if you follow these six steps, you should come out with more money in your pocket. In the end I found a DIRECT flight to London round trip for about $665 on Virgin Atlantic. Other sites also have hotel + flight package deals that are great to look into, but I luckily get to stay with my old roommate for free for two weeks. I will be in London from June 2nd-16th :)
Here's are some links you might want to look into when booking your next European flight!