Good question! The trick is to apply an even amount of pressure from behind with the thumb (as you noticed, in the middle of the neck) and in the front with the partially barred ring finger. This will help to keep the neck steady. Furthermore, make sure you are holding the ukulele properly and using form 1. Also, read over form 3 to get a better understanding of why it’s important to apply an even amount of pressure.
Give this a shot and let me know how it goes π