I always loved the idea of a diary. I tried to keep one several times as a child, and never succeeded past a couple of weeks. Then, my beloved sister introduced me to the 10 Year Journal, a journal that only allows you four lines per day. Each page is dedicated to one day of the year. For example, the first page is January 1. The first four lines of it are for 1/1/06. Beneath that are four more lines for 1/1/07, and so on.
To my surprise, I have been able to keep with this format for over five years now, only missing a few days here and there. It's terrific to be jotting down the day's events and be able to look back and see what was going on last year on the same day, or even three years ago, or four. I've heard of people giving it as a wedding gift so the newlyweds could keep track of their new life together.
It's so easy to keep up with it since it only takes a couple of minutes at night. I highly recommend it, newlywed or not. :)