What helps me is warming up with no stirrups and really thinking about your legs being pulled down into the arena sand. Another thing you can do is take the leg that tends to go forward, flex it up and back (so your knee is pointing down) and hold it up by your ankle. It will stretch and open up your pelvis so you can stay in position. Just don't overdue it.
And you're sure the saddle isn't playing a part in it? It could be asymmetrically 'level' like one of your seat bones is slightly higher up than your other one.
Also, some work in two point gets your legs underneath you.
Best of luck, congratulations :)
"Strength is the ability to use a muscle without tension"