1) Get an empty notebook and write all the funny things your kids say. My father did this with a simple memo book. As we gor older, he would pull it out and read entries to us after dinner. I started a book when Natalie was two and a half and just starting to say all sorts of cute and funny things. I try to write them down as soon as they happen, or I forget. In our house, the saying "that needs to go in the book" will live on.
2) I can't remember where unread this, but way back when Natalie was an infant, I read the advice to not just take pictures of your child, but also their things. One of my favorite things to photograph is Natalie's toys. I love the creative arrangements of her Little People dolls or the birthday parties her big dolls. I'm excited to see how William plays as he gets older and capture pictures of that as well. Currently, he is starting to make some real fun block towers.