I also endorse flightmemory. I actually use the German version (flugstatistik.de) as that was the only version available when I started using it. (Yes, I know you see the same data in the English version but habits are hard to break.) I never log tail numbers, though, or actual flight times, only scheduled. Link is in my signature.

Emails are a good source for building the history. That's how I backtracked a bit when I first started recording. I also have some paper printouts of old frequent flyer program updates (remember when those came by mail?) that I still haven't input. You can at least get dates and routes from those. And if you're into this enough to ask the question you may be someone who saves itineraries and boarding passes from old trips.