Persistence is definitely key. Check at least once a day, (twice at most to keep yourself from procrastinating too much,) even after you have booked. Example: I just booked MSP-LHR roundtrip for this summer, saver J one way and saver Y the other. But originally I could only find the saver J, and had to get pass Y on the return leg. But after keeping the search going, a couple days later I found that I could return a day early and get saver Y because a seat had opened up, so I bit the $25 change fee and saved myself the 25k miles.