Try no stirrups over small jumps or tie them on the flat for muscle memory:)
I compete in the 1.35m competitions in South Africa :) the first thing you need to do is make sure that your stirrups are at a correct length (too long will cause your leg to swing forward and as a result your seat will be unbalanced and your legs will swing back and forth the whole time during your course). Then practice at home by pushing your heel down as far as possible while jumping and squeezing slightly more than usual with your calf, keeping your knees open. Give it a try, it works for me in the equitation classes with the bigger fences :)
I personally haven't jumped but I've heard Millbrook Stirrup leathers work like magic to keep your leg locked in place, especially for the jumpers.