sub-page settings saving in NETWORK DASHBOARD

The easiest thing to do is to set the form’s action='' and use the global $plugin_page; as described in the Notes section of add_submenu_page().

For example:

add_action ('network_admin_menu', 'add_network_menus') ;

function
add_network_menus ()
{
    add_menu_page ('My Page Title', 'My Menu Title',
        'manage_network', 'my_menu_slug', 'my_network_admin_page') ;
    add_submenu_page ('my_menu_slug', 'Some Subpage Title',
        'My Submeny Title', 'manage_network', 'my_submenu_slug',
        'network_admin_subpage') ;

    return ;
}

function
network_admin_page ()
{
    global $title, $plugin_page ;

    echo <<<EOF
<div class="wrap">
    <div id='icon-tools' class="icon32"></div>
    <h2>$title</h2>

    <p>more content here</p>
</div>
EOF;

    return ;
}

function
network_admin_subpage ()
{
    global $title, $plugin_page ;

    echo <<<EOF
<div class="wrap">
    <div id='icon-tools' class="icon32"></div>
    <h2>$title</h2>

    <p>more content here</p>
    <form action='' method='GET'>
        <input type="hidden" name="page" value="$plugin_page" />
        <input type="submit" value="Submit">
    </form>
</div>
EOF;

    return ;
}

This works for both normal and network admin menus.