Do some waist/hip drills that many boxers and TKDists do.
What type of spinning kick is it? If it's a back kick, or a spinning hook kick, just try working on the hook part. Then do it.
I don't know what I did, I just practiced and practiced and I got my balance, speed, agility down.
One thing on posture is MAKE SURE YOU HAVE PERFECT technique. A spinning kick is graceful, quick, and accurate it shouldn't be awkward. For me, what I like to do is lean back a bit for an extra reach and to have a bit more balance. And for back spinning kicks, leaning back is very important. You can get more power, more reach, be better balanced, and a bit more accurate. Some may argue that if you don't lean back, then you'll have more control... Not true, because your body will be in an awkward twist that way, you want your body to feel natural. And when you lean back, make sure you are aligned with your body. If you face a mirror and do a spinning kick, your you really shouldn't be able to see your upper body that much other than your head. Your upper body shouldn't be leaning to the left or right for spinning kicks that are above the waist.
Another tip is that your upper body plays a huge role on how your lower body moves. You need to use your upper body for momentum and balance. When you execute the kick, make sure your hands are up and close to your body. They shouldn't be way out there all over the place.
But one important thing... I would suggest you not throw a spinning kick in real life. Even if you see a HUGE opening, it is not recommended because if you perceive wrong, then you run huge risks.