Remember that merged documents that you save are separate from the main document. It's a good idea to save the main document itself if you plan to use it for another mail merge.

When you save the main document, in addition to its content and fields, you also save its connection to the data file. The next time you open the main document, you're prompted to choose whether you want the information from the data file to be merged again into the main document.

• If you click Yes, the document opens with information from the first record merged in. If you open the task pane (Tools menu, Letters and Mailings submenu, Mail Merge command), you are at the Select recipients step. You can click hyperlinks in the task pane to modify the data file to include a different set of records or to connect to a different data file. Then, you can click Next at the bottom of the task pane to proceed with the merge.
• If you click No, the connection between the main document and the data file is broken. The main document becomes a standard Word document. Fields are replaced with the unique information from the first record.

http://office.microsoft.com/en-us/help/HA011095521033.aspx