I gave this question a lot of thought and looked back at the way we preserved memories of our three babies and to be honest the best ways that come to mind are not fun, nor creative, but they preserved the memories for me.
1) Doing stuff with them and talking about it with each of the other kids as they get older. Blogging is one way that has helped me remember funny situations or unique milestones. On the weekends when I spend the majority of my time with the kids we chat a lot over meals and around fun and games and I tell them stories about how they were as kids, or funny things they did too at that age. It's one of the main reasons I'm continued to blog - my blog started around the birth of child #1 in 2004.
Now that my oldest 2 are reading I let them read the posts that relate to them and it makes them excited to read about themselves while working on their reading. It's funny because when we first started this, my middle child - then 4 - asked me why do they call me the "Urban Daddy". So I told him. I'm a daddy and we live in urban-Toronto. "Oh" was his response. Obviously not what he was looking for.
The other thing that we did not do enough of was to take video! We've been watching it as a family recently on our TV and it's amazing how alike they were at the same age and their progress they made walking, talking and clothes they wore when they were all, say 18 month. It's cool and makes me kind of sad that I don't have much from when I was that age... Well I do, but it's on reel to reel (Yes, I'm old!).