Salt, especially if you're increasing your water intake. Salt boosts BP, that's why folks with high blood pressure have to go on low salt diets. Elevating the head of your bed can help in the mornings, eat 6 or 7 small meals rather than three big ones, be careful of hot water, showers, spas, etc.
And if you feel like you're going to faint - no matter where you are - LAY DOWN. You've got to get the blood back up to your brain or you'll have (like I did) an anoxic seizure, that's a seizure caused by lack of oxygen.
There are prescription medications you can take but they all have side effects and besides most folks your age outgrow low BP.