Here is something that I have found to help me with stretching my legs.

Start by standing next to a wall or poll or anything to hold onto really. Then start one leg in a swinging motion in front of you. Let your leg come back down and swing behind you. Kind of like a pendulum motion. Try to let gravity do the work.  Try to help your legs reach further and further in very small steps. Obviously do the same to both legs.

You will find that you will be able kick higher and higher as your muscles stretch.

The pendulum excercise works well as a warm up for stretching.  Yoga is a great way to increase flexiblilty.  Even if you only do it once or twice a week.