Yeah I kept pretty much all my FF statements, and fortunately my intense travel started about the time that the FF programs came along. During the 70's I didn't travel all that much so was able to remember most flights by approximate year and route and often by airline. The big challenge was reconstructing all of the free domestic travel (it was easy to remember international). Fortunately, my #1 carrier was nice enough to print off 10 years of history which included the free trips. Another thing that was a big help was hotel statements and business calendars. So if I was looking at the flight data in date order, and I was a misssing flight or there was a week with no travel at all (very rare for me), I could look at the other sorces and come up with a best guess.