I stopped gripping with my knees!
On the subject of posting bareback -
I found it was actually easier to keep my leg long and straight during that because there wasn't any stirrup to tempt me into bracing against it. Where I run into trouble is when the stirrups are there to tempt me.
Ah, precisely. It's not done by gripping of the knees, but with thigh and buttock slight tension, and thank you for the reminder about leg position. I completely forgot it. Long and straight, knee turned in slightly to bring more thigh contact.
Interestingly, though I don't advise it, a very short leg, deeply bent, like a jockey can make posting easy too, but that's more of a knee pinch, unstable without stirrups, and brings the ischia once again in hard contact with horses' backs.
The leg position in fact depends, if one is to have a secure and strong balanced seat, on the same thing as the seat in a saddle. Heel, hip, and shoulder all lining up with the ear in a plumb vertical line.
The only relief (as this is not a comfortable position on many horses bareback) is that slight tension of thigh and buttocks.
Donald, Altea, and Bonnie Cupcake