in addition to keeping your tanks as much as possible try keeping them half empty and if you can set the airflow to closed (turn the ring between airflow settings so the holes are as blocked off as much as possible,) to help seal the tank. The first time I flew vaping I had filled my tanks full and just put them in a plastic bag in my carry on and they were nice enough to leak all over the bag and almost empty themselves completely (thank God my mod wasn't connected inside the bag.) on the return trip I left my tanks half empty and closed the airflow off and left the airflow open on the other and again just threw them in a plastic bag in my carry on,,,,the one I closed off the airflow on didn't leak this time while the other one did, but not as much.
These tanks work on a combination of being vacuum and gravity fed....so drastic changes in temperature and air pressure from altitude can cause them to dump their contents into a coil quickly flooding them and causing them to leak out. By leaving the tank half empty and closing off the airflow you're sealing in the air in the tank creating it's own pressure seal and as long as it's at least half empty the pressure in it shouldn't be enough to flood it, especially stored upside down...at least that's my best guess...I just know it worked for me
I never claimed to know all the science behind it.
I hope this helps...best of luck