Remember to use your arms. Swing them wide and up to gain as much height as you need.
Kicking with an inside crescent instead of a round kick is possible too but is quite a bit harder since your hip will be facing forward a lot longer, making it more difficult to land on the kicking leg. Check out Jujimufus 540 Kicks to get an idea how it would look like.
Many beginners will instinctively look at the ground when they learn this trick. Try not to. Spot a target in front of you and kick through it. This applies to any trick involving kicks.
Drill your tornado kicks if the 540 is giving you trouble. Make it higher and faster. It also helps to learn a Tornado Turbo, which is essentially a Tornado Kick landed on both feet.
Do not travel. A good 540 Kick should almost be stationary. When you come down for the landing, try to get your foot directly underneath you to lessen the impact. If you land with your center of gravity off, you will likely fall or have a very hard landing, which can cause injury.