Well, I'm not the right person to answer this because I'm trying to get better on panning as well
But I got some tips that may help you too:
1- Hold your breath while panning. It helps you not to shake.
2- Try with IS on and off. See what works best for you and use it. You may find IS helps or difficult your shots.
3- A good technique I learned that can really help:
- If you're right handed, hold your camera firmly with the right hand
- Put your left hand on your right shoulder
- if your lens are long enough, you can rest it on your left elbow
I think this technique works better for me than holding the lens with my left hand.
Besides that, it's all about practicing, practicing, practicing. A bit of luck can help you a lot too