I have most of my FF statements dating back to 1981. Where I'm missing them I'm using my old calendars plus hotel statements to estimate where I was and how I got there. The hardest thing to figure out is free trips because they're generally not on the statements. I've also found it useful to view all of the flights I've already entered in date order. Then I can see that maybe I flew to ORD, and maybe the next flight is out of LGA, so I know I'm missing the ORD-LGA flight. So the first thing I do is check the statements to see if I made an error, and if not I then do a best guess, typically assuming it's on a likely airline for which I don't have a statement.