Work on your flexibility. One of the main factors of executing and developing a high, properly aligned extension is the flexibility of your leg muscles. Your natural flexibility will determine how high your extension will go, although there is much you can do to achieve the highest extension within your capabilities. Cross training in forms that develop functional flexibility such as yoga, Pilates and Gyrotonic can help improve the flexibility of your leg muscles. If you are not a ballet dancer, consider taking a ballet class at least twice a week. Jazz, contemporary and ballroom dancers may all improve flexibility with ballet study.
Develop your strength. Flexibility is important in reaching a high extension, which will be evident when holding your foot with a stretched leg, or in a grand battement (high kick.) However, you must also have the strength to maintain the extension with your leg in the air. Though your quadriceps (the muscles on the front of your thighs) will want to do the work to extend your leg, you must work with the hamstring muscles (the muscles that run down the back of the leg).. Your flexibility and alignment must work in harmony if you want to raise the level of your extension. The height of your extension won't improve if you "cheat" or achieve the extension with improper alignment. As you extend your leg to a second position extension, (to the side), initiate the move from the hamstrings rather than the quadriceps.
Stretch on a ballet barre. Ballet barre stretches can help you maintain your balance while developing strength and flexibility. Stretching your leg on the barre in different directions can also help you maintain proper alignment. Place your foot on the barre to your front or side, with the foot pointed and the leg straight. Keep the hip down and your body will "memorize" this alignment and it will be easier to recreate the stretch without the barre.