Here is a Kung Fu warm-up and
stretching routine. Incorporate the bits that you find useful in
your Taekwondo stretching program.
stretching page for written and photographic instructions
regarding a wide variety of Taekwondo stretches.
My favorite Taekwondo stretching book is
Ultimate Flexibility: A Complete Guide to Stretching for Martial Arts.
Hundreds of great stretches (with pictures). Or check out these
other popular stretching books.