Stretch on a regular basis. Make it a point to incorporate stretching as a part of your daily regimen, the longer you get your muscles, the easier it will be to extend. Focus on the muscles you need most to perform the kick such as your hamstrings or hip flexors.
Warm your body with a pre-stretch workout. Walk around briskly for 5-10 minutes to warm your muscles in preparation for the stretches. Avoid stretching without warming up to avoid of injury. Stretch your entire body to warm and prepare all of your ligaments for a workout.
Learn to breathe through your stretches. Exhale out when you're going deeper and inhale to give your body a chance to assist you in going longer. Control your breathing to help your muscles get the oxygen they need.
Take your time when stretching. Avoid rushing through. Give yourself weeks, even months before you see a dynamic difference in your lengthening. Listen to your body and stop when you're going too far.
Add strength training with your stretching routine. Building your muscles in other areas to hold and balance your body when you are executing your splits will be useful when lengthening your kicks.
Use the butterfly stretch for hip flexors. Sit on the floor with your back as straight as you can, fold your legs in so your feet meet. Flutter your knees up and down slowly feeling the stretch in your upper inner thigh or hips. Stretch legs out to give a rest and go back into position.
Loosen inner thigh muscles for side kicks. Stand with your feet hip width apart. Slowly slide your feet away from each other going into a side split. You may need a support bar or to place your hands on the floor if you are not yet limber enough to accomplish the side straddle split fully.