Menu highlighting – keep parent page current

Hi there. 🙂 I am using dynamic menu highlighting on a project, listing four specific pages plus the blog as menu items. I was wondering if someone could give me a pointer on how to keep the pages listed in my menu highlighted as my visitor browse their sub-pages. Using if (is_home() || is_category() || is_single()) I have managed to keep the “Blog” menu item highlighted when visitors are viewing single posts or category archives, but the sub-page problem has got me stumped. Any help would be much appreciated!

A Merry Christmas to you all,
Birgit