WCF/SOAP: Move XML namespace definition to parent element

I use WCF to implement a client for a SOAP web service. The code is generated by "Add Service Reference" based on the WSDL file.

The elements of the SOAP request are defined in different namespaces. Now, a sample message looks like this:

<s:Envelope xmlns:s="http://ift.tt/sVJIaE">
   <s:Body xmlns:xsi="http://ift.tt/ra1lAU"  
           xmlns:xsd="http://ift.tt/tphNwY">
      <myMethod xmlns="http://ift.tt/1Fx4DIj">
         <RequestData>
            <Foo xmlns="http://ift.tt/1Lgixib">some</Foo>
            <Bar xmlns="http://ift.tt/1Lgixib">thing</Bar>
            <Baz xmlns="http://ift.tt/1Lgixib">content</Baz>
            <!-- .... more elements .... --> 
         </RequestData>
      </myMethod>
   </s:Body>
</s:Envelope>

which contains the redundant definition of the elements namespace. In order to reduce message size, I would prefer to have the namespace definitions at the top of the message, eg. like this:

<s:Envelope xmlns:s="http://ift.tt/sVJIaE">
   <s:Body xmlns:xsi="http://ift.tt/ra1lAU" 
           xmlns:xsd="http://ift.tt/tphNwY" 
           xmlns:a="http://ift.tt/1Fx4DIj" 
           xmlns:b="http://ift.tt/1Lgixib">
      <a:myMethod>
         <a:RequestData>
            <b:Foo>some</b:Foo>
            <b:Bar xmlns="http://ift.tt/1Lgixib">thing</b:Bar>
            <b:Baz xmlns="http://ift.tt/1Lgixib">content</b:Baz>
            <!-- .... more elements .... --> 
         </a:RequestData>
      </a:myMethod>
   </s:Body>
</s:Envelope>

(I don't really care which parent element defines the namespaces, so any way to move the namespace definitions to Envelope, Body or MyMethod would be okay for me.)

How can I achieve this?