Every January, I go through my perpetual calendar (these are very nice - you always have a backup of all those important dates in one easy to find place) that lists all anniversaries and birthdays. I then add those dates to my planner. I also add a note about 2 or 3 weeks before to remind me that a birthday is coming up, so that I can have enough time to find something and mail it (no more belated cards!).
I also use a calendar on the fridge. We're always in the kitchen, so I put the calendar where we see it everyday. This calendar lists all the important events that either involve me, my spouse, or both of us - that way, no more questions about: "oh, we have that?" "what time is that thing happening again?" I also plan my meals for the week and write them on there. That way, we both know what the plan is for the week in terms of meals - no more standing in the pantry dumbfounded and then giving in to pizza. Again. The Container Store has this great magnetic calendar that would be perfect for this use. Should have gotten it when I saw it :o)
Finally, and probably most importantly, take the time in January to go through all your documents and see if you have anything that needs to be renewed in the coming year. Write down if and when you need to renew your driver's license or your passport. You won't have to do it right then, but at least you won't get caught by surprise.