These stirrups are so cool...
MDC Sport Classic Stirrup - Stirrup Irons from SmartPak Equine
I just got these mdc stirrups. They were a pretty penny, but they are fantastic. I honestly don't get how they work, but much like a car with 4WD that keeps you connected to the road, these stay glued to your feet. I guess they make micro adjustments every time your weight shifts, but don't really care because they are so cool. I have never before been able to ride around in half seat at the trot without having to use my horse's neck for the occasional rebalance. With these stirrups, my balance is fantastic. I haven't needed a neck check yet. If anyone else has been struggling with the ever shifting stirrup problem, you might want to some money at these.
You just have to see your distance...you don't have to like it.