Hi Hannah,

Windscreen wiper legs are difficult to fix, but it can be done with a little work.  First of all, are your stirrups the correct length for your conformation?  I have a student who has very short legs and I have her ride with shorter stirrups, even though her horse is over 16 hands.  Her riding teacher at her College wants her to ride with longer stirrups so she "looks" better for the intercollegiate competitions, but the result is, legs that will not stay still.

I think you are using your stirrups too much, in other words, relying on them to get you up and down at the rising trot.  Posting while rising, without stirrups is most certainly the answer....it isn't fun, but it can be done and it will get increasingly better with practice. When I was younger (in my mid-30's, I had an instructor who made us all ride stirrupless.  I could go round and round the arena many times without tiring because I was strong.  My legs were glued to my horses sides.  I rode about 6 times per week, both in the arena and lots of cross-country.  I also did aerobic exercise 3 times per week.  Fitness is very important to good riding, both for horse and especially the rider!  Your entire leg needs to be fit so you can embrace the horses barrel with your entire inside leg, from upper thigh down to the ankle...this does not mean pinching anywhere, just a nice hug around your horse.

I hope this helps....happy stirrupless riding!!!  OUCH!!!