For me, I had the worst stirrup-slip/losing issues when I was pinching or collapsing somewhere and the weight couldn't get down the back of my legs.
The best thing that helped me was warming up in two point. Start at the walk.. grab mane and just do lots of figures and then when you feel secure transition to trot and warm up in trot as well.
This forces the weight down your legs. Whatever you feel is how it should feel once you are sitting down usually as well. That long grip down your leg feeling.
"Strength is the ability to use a muscle without tension"