1. BE FLEXIBLE
Select the flexible date option if you don't need to be at the destination at a very exact time. This will allow the search engine to show you what are the cheapest dates. Sometimes even a day can make a difference. Another trick is to also select nearby airports. Some airports may ask the airlines for lower taxes which means lower prices for you too.
2. BOOK IN ADVANCE
I know you know this already but I cannot stress it enough. With some exceptions this is still the golden rule of thrifty travels.
3. CLEAR COOKIES
I heard people say that search engines remember what you looked for and they will show you higher prices next time you check. I thought it was a myth! But I was recently searching for flights from London to Chicago and they were around £500 and when I checked a few days later they were over £550. Clear ALL cookies before every search - clear cookies even if you are searching in incognito mode. I am serious.