JavaScript for Disabling a field once its contains a value in crm 2011

 Write this javascript in onload event of a form

     if( Xrm.Page.data.entity.attributes.get('new_consultant').getValue() !=null)
     {
         var AddressType = Xrm.Page.ui.controls.get('new_consultant');
         AddressType.setDisabled(true);
     }