With the time I have found I have become more and more nervous about flying - never used to be. So now have to take measures when flying or I am a wreck by the time the plane lands.
Personally I'm not keen on taking loads of meds so I tend to see if I can get a night flight so I know I'll sleep most of the way or if I can only get a day flight, I will stay up the night before so I'm knackered and end up sleeping.
I have also used Kalms, herbal Nytol, Bachs rescue remedy - although not all on the same occassion.
Why don't you do a trial run at home of what you decide to take. If you've never tried Valium before you may get freaked out by being in an unfamiliar place feeling the effects of the Valium. With any meds I think it's always nice to try at home first so you know what side effects you may encounter (if any) or how it makes you feel.
Good luck with the flight, I feel for you