Since Custom-Fields are meant on a per-post scope, I would recommend using the Options meta-data store instead. This way you can change the values from the Admin panel which is mush more convenient and consistent.
This is a tutorial showing how you can add this functionality to WP
//Custom Theme Settings
add_action('admin_menu', 'add_gcf_interface');
function add_gcf_interface() {
add_options_page('Global Custom Fields', 'Global Custom Fields', '8', 'functions', 'editglobalcustomfields');
}
function editglobalcustomfields() {
?>
<div class='wrap'>
<h2>Global Custom Fields</h2>
<form method="post" action="options.php">
<?php wp_nonce_field('update-options') ?>
<p><strong>My Name:</strong><br />
<input type="text" name="myname" size="45" value="<?php echo get_option('myname'); ?>" /></p>
<p><strong>Amazon ID:</strong><br />
<input type="text" name="amazonid" size="45" value="<?php echo get_option('amazonid'); ?>" /></p>
<p><strong>Today's Featured Website:</strong><br />
<input type="text" name="todaysite" size="45" value="<?php echo get_option('todaysite'); ?>" /></p>
<p><strong>Welcome Text:</strong><br />
<textarea name="welcomemessage" cols="100%" rows="7"><?php echo get_option('welcomemessage'); ?></textarea></p>
<p><input type="submit" name="Submit" value="Update Options" /></p>
<input type="hidden" name="action" value="update" />
<input type="hidden" name="page_options" value="myname,amazonid,todaysite,welcomemessage" />
</form>
</div>
<?php
}