Flexibility in Taekwondo

Time to admit something. The thing I enjoy the most about Taekwondo is the flexibility exercises. I'm lucky in this, being naturally flexible. Most people don't have such luck, and flexibility can be a major sticking point. To execute a side kick (dwi chagi) effectively at waist height or above, you need to have reasonably flexible hips. Side kick is probably one of the most effective techniques Taekwondo teaches - do it right and your opponent/attacker will go down and not get back up (at least not in a hurry) and it's based on flexibility at the hip.
If you're not taking cod liver oil yet, start now. It's great for maintaining joint flexibility, and strength in your bones.
Stretching exercises shouldn't hurt, but you should feel a slight pull. If you don't push slightly beyond your current limits, you won't improve. To understand what is going on from a biological point of view when you stretch:
Voluntary muscles are elongated fleshy sacs. As a result of nervous stimulation, they contract and produce movement Their force is transmitted through inelastic strips of specialized connective tissue (tendons). The ligaments support the joints and are made up of a more flexible form of connective tissue. The tension condition of the muscles & tendons is monitered by proprioceptors, which are highly specialised structures found in both muscles and tendons. They are comprised of a great many nerve endings ( hence the feeling of pain when stretching beyond their current physical limits).
The important thing to be aware of is that your tendons, right now, have a natural limit on how far they can be stretched or extended. This is based on your current lifestyle, and genetic characteristics. You can't change your genetic makeup, but you CAN alter your lifestyle, and most importantly, the amount of effort you put into your stretching. If you really want to improve flexibility:
1) Stretch EVERY DAY. Initially, touching your toes every morning if that's all you have time for, but do more when you can and get into a regime where you perform 3-4 different streches every day.
2) Push a LITTLE each time you stretch. Pushing it to far can result in temporary, or even permanent damage. The nerves in your proprioceptors will let you know when you are going slightly beyond your current limit, and will scream at you when you go too far. Listen to what they are telling you!
3) Perform some stretches after any physical activity. I do quite a few stretches at the end of a Taekwondo lesson. It's a great time to do it as your muscles are very warm, and succeptable to that extra stretch.