Start looking as soon as you can, even a year out. You must have price in mind that you believe is a good or great price. When you find that price, book it immediately. If the price goes lower as you get closer, see if it is worth eating a $50 or whatever the cancellation fee is in return for buying a new ticket. And second, airlines do this all the time but it is your out. If they chance the time on a flight, even by a few minutes, they will notify you and give you 30 days or so cancel your flight - at no charge. You can use those tricks to get yourself a good fare. We purchased our Dec 2017 Holiday fare to London in mid-July. We saw the price at $588 r/t from SFO so in our experience, that was like $400 less than what we considered a good deal (anything under $1K from SFO - LHR is good but we were looking for under $700 to book as one year earlier we found a $688 flight during Easter break). Good luck. No guarantees with fares but if you know what a good fare is then you have a better chance at getting it or something close to it.