Remember what gifts your wife has gotten you.
This is a simple rule with two major applications. First, remember the gifts your wife has given you over the course of your relationship. This sounds simple but it really is not once all of the Christmas, anniversary, and birthday gifts start to add up over the years. Still quantity is not an excuse for forgetfulness. When your wife asks you what your favorite gift that she got you is, an answer like "I love them all" will not cut it.
The second application is to remember what gifts your wife did NOT get you. It is never good to attribute to your wife a gift that she did not get you. This is particularly important if the gift came from an ex girlfriend.