Sodium is number one like everyone before said.
One thing that is often over looked is vasodilators that can help bring your BP down over the long haul. If you have hypertension or high cholesterol this really isnt a fix but can help if you don't have those symptoms.
Caffeine is a vasodilator but raises HR so not good but try supplementing a daily vitamin with 1000mg L arginine x2 a day and healthy dose of Niacin. Just a thought, you can get that stuff pretty cheap at any drug store.
Not really a good (or legal) solution for aviators.
Brett