You could also just say " please pop your ears." "Be sure to pop your ears while the plane is descending."
Barb, how did you happen to know "Valsalva maneuver"? I don't think anyone I know, except for doctors, would know that term. I only learned it recently, when I lost the hearing in one ear and also had the persistent feeling that I could not "pop" that ear. The ear doctor told me that the ear was in fact popping, but I couldn't hear it! It still feels strange.
I would be very interested to know what "popping your ears" is called in other languages -- not the medical term, but the everyday term -- and how it translates literally into English.