I have an existing XML file that I would like to append without changing the format. Existing File looks like this:

<Clients>
  <User username="farstucker">
    <UserID>1</UserID>
    <DOB />
    <FirstName>Steve</FirstName>
    <LastName>Lawrence</LastName>
    <Location>NYC</Location>
  </User>
</Clients>

How can I add another user using this format? My existing code is:

        string fileLocation = "clients.xml";

        XmlTextWriter writer;

        if (!File.Exists(fileLocation))
        {
            writer = new XmlTextWriter(fileLocation, null);
            writer.WriteStartDocument();

            // Write the Root Element
            writer.WriteStartElement("Clients");

            // End Element and Close
            writer.WriteEndElement();
            writer.Close();
        }
// Append new data here

Ive thought about using XmlDocument Fragment to append the data but Im not certain if I can maintain the existing format ( and empty tags ) without messing up the file.

Any advice you could give is much appreciated.

EDIT Ive changed the code to read the original XML but the file keeps getting overwritten.