For my first, I had high hopes to do the scrapbooking, baby book, etc. thing. Fortunately, he was a grumpy baby who left me no time to do any of that. I say fortunately because I feel like what I did for him would have to be done for my other children and I KNOW I don't have time now for all of that.
What I do is try to keep up with my blog...someday I'll probably print it out for each of them. I also try to stay on top of taking photos and creating a slide show for each 2 year span for each child (does that even make sense?). I just put the digital pictures in a folder for that child as I upload them and then before the even # birthdays go through and pick the best/most significant ones for the slideshow. Then I put it to music and burn it.
I also get a memory box for each baby and keep some of their 'treasures' from their first year in there. The hat from the hospital, foot and hand prints, a birth announcement, cards given to them, a list of milestones (first foods, words), etc. Finally, I keep hard copy stuff (drawings, artwork, etc) in a filing cabinet...a folder for each year for each kid.
Whatever you do will be appreciated by your little one when he/she grows up. Don't put too much pressure on yourself.