The research says the proper way for you to ride is to not hold your breath or clench your teeth, but to breath in and out in medium and continuous breaths all the way through the ride. However, the research also says to avoid inversions. You can buy personal blood pressure monitors and check yourself on smaller coasters to see what the effects are, then gradually build up to megacoasters. There are people riding coasters who have had heart attacks. They're riding the Blue Streak, Beast and Raven, but not Phantom's Revenge or the other steelies.