If you want to declare entities, you MUST do so within the document DOCTYPE declaration that always follows the prolog (DTD and xml Declaration) and looks like the following:

    <?xml version="1.0"?>
    <!DOCTYPE myDocument [
    ...here is where you declare your entities....
    ]>
    <myDocument>
    ...here is the body of your document....
    </myDocument>

Thus, you might have something like the following (Consider how much easier changing office addresses is when you use entities!):

    <?xml version="1.0"?>
    <!DOCTYPE CLIENTS [
          <!ENTITY ninthFloorAddress "2345 Broadway St Floor 9">
          <!ENTITY eighthFloorAddress "2345 Broadway St Floor 8">
          <!ENTITY seventhFloorAddress "2345 Broadway St Floor 7">
          ]>
    <CLIENTS>
    <CLIENT>
    <NAME>Barnaby Hazen</NAME>
    <ADDRESS>&ninthFloorAddress;</ADDRESS>
    <PHONE>x345</PHONE>
    </CLIENT>

    <CLIENT>
    <NAME>David Florey</NAME>
    <ADDRESS>&ninthFloorAddress;</ADDRESS>
    <PHONE>x111</PHONE>
    </CLIENT>

    <CLIENT>
    <NAME>Danette Ellsworth</NAME>
    <ADDRESS>&ninthFloorAddress;</ADDRESS>
    <PHONE>x346</PHONE>
    </CLIENT>

    <CLIENT>
    <NAME>Rachna Dhamija</NAME>
    <ADDRESS>&seventhFloorAddress;</ADDRESS>
    <PHONE>x289</PHONE>
    </CLIENT>

    <CLIENT>
    <NAME>Kristin Mancuso</NAME>
    <ADDRESS>&eighthFloorAddress;</ADDRESS>
    <PHONE>x945</PHONE>
    </CLIENT>
    </CLIENTS>