Well, that was fun! Like many others have said, it's hard to choose how far back to go. I decided to start around the time I had my first child - my records from before that time were just too spotty. It shouldn't be too hard to keep up with the data now, though.
I also created separate accounts to enter all of my daughter's and son's flights. If flightmemory is still around (I hope so!), I think a poster of each of their travels will be a cool present for their 5th, 10th, etc., birthday. I've finished the ones for my daughter so far - she's gone 108,756 miles in under 4 years. Probably not the most impressive amount of flying for a FT baby, but still not too bad!