Bright eyes, are you a hunter/jumper by any chance?
Usually we eventers get comments on "hunching" from hunter riders (and a lot of other things about jumping position, which aren't unfounded, I'm afriad...
). It is actually a pretty normal position for cross-country, to have your backs slightly rounded, as it is just exhausting to try to ride a 2-mile plus cross country course with a ramrod-straight back. If you think so, though, I will work on that, and the perching too...I can kind of see what you mean on that one. I think it goes back to what My Boy Puck said about pinching with my knee.
I'm pretty sure my saddle fits...I think it is just me who is not used to using my legs that way. I'll try the balance thing, though.
So overall, just no-stirrup and balancing work seem to be what I need, right? Any other suggestions?