As an Executive Assistant being organized is ever so important. Organizing emails using color categories is a very useful tool in Outlook 2007.  I use this feature to group emails and then later pulling them by color.

Go to Color>All Categories. A new box will open up, label the colors. For example, each time you make a purchase, assign the yellow color.

Then create a search folder for the yellow color. Next time when you need to search for expenses incurred for the year, click on the search folder and all the messages assigned the expense category will show up! No more printing and filing receipts!