Do you have any tips on how to keep your lower leg still and prevent it from slipping too far behind you ?

When trotting try standing up in the stirrups and then sitting back down. If you can stand up and balance then your legs are in the correct place.
I agree with @sprintdog, tie your leg to the girth with bailing twine, awesome results!
If you look on my wall, I just wrote a response to the same question but in short, try and focus on gripping with your lower leg and supporting your seat with your hamstrings. If you grip with your knees at all, your leg will swing. When in doubt, do no-stirrup work! No stirrup work is always the answer lol, hope this helped!
no i havent, but i will try it thanks!
Have you tried tying your stirrups to the girth with a spur strap? It really helps!
Join the fun and sign up to connect with our 200,000 members!