Altitude sickness is extremely dangerous - people die from it - don't take it lightly. If you feel symtoms of altitude sickness, immediately go to a lower elevation.
The only natural preventive I've ever heard of is acclimization. Do this by climbing higher, in stages, than you will be spending the majority of your time. For example, the first day climb, or drive, (if you drive, gain and give up elevation slowly) to 7000 feet and spend a couple hours, then return to a lower elevation. The next day go to 8500' for a few hours and return to 7000' for the night. COntinue this progression and you should lessen the chance of altitude sickness. A few years ago we climbed Kilimanjaro in Africa (19,500'), and each day we would hike up to an elevation higher than where we slept. On the third night we camped about 15,000', then hiked to the summit the next morning, and had little problem with altitude sickness, but man, it's hard to even think in the thin air @ 19,000'!
As others have said, drink lots of water.