Personally, I've already bought my Europe tickets for May of next year. Airlines work similar to hotels and as the demand increases, you'll pay more for less choices (unless you luck into something). When I went to Hawaii in 2011, I found a great airfare and snapped it up ASAP. Three days later that fare was no longer available, (it nearly doubled) and I've not seen a flight to Hawaii that low since then. For my trip to Europe in July 2012, made the mistake of waiting, hoping prices might go down (since I had plenty of time- started planning in Oct 2011) and they went up by over $500 dollars a ticket just before Christmas- and stayed there. I've been tracking Europe fares for the last 6 months this year and they haven't fluctuated more than forty bucks in all that time. I decided to err on the side of caution and buy my flight while I still had choices of times, layovers, layover cities, etc. The price for my ticket is cheaper than what I paid in 2012- by $300. However, ticket prices for after mid May are much higher- after school gets out and kids start travelling.
So my two cents are- if ya know when and where you want to go, and you have the money- why wait? Prices might go down, but they're much more likely to go up!