another option is to use the following code for the forms action:
action="<?php echo $_SERVER["PHP_SELF"]; ?><?php echo (isset($_SERVER["QUERY_STRING"]) && $_SERVER["QUERY_STRING"] != "")?"?".$_SERVER["QUERY_STRING"]:""; ?>"
this will ensure that any URL variables passed in are submited with the form, then in the insert / update server behaviors, make sure the "Pass Original Querystring" option is checked.