There are so many variables it's hard to pinpoint what EXACTLY causes high blood pressure.
Exercise, meditate, relax, breathing exercise,
Garlic, Hawthorn(no, not the Harry Potter School, it is an Herb), Soy Greens, Collared Greens, Coco powder, Olives, beat juice, Whey Protein(found in fitness shakes, it's pretty good. It contains glutathione which helps anti again & preventing cancer#, & LEGAL(I repeat, LEGAL, hence there are stores that do sell it legally depending where in the world you live) Mary Jane(but ONLY if you are AT LEAST 18!!! * take care, bcuz the legal stuff has proven to be more dangerous, so pace yourself if you've not ever done it before is about all I can say-I've not ever done it so I really can't give you pointers there).
NOTE: Not to replace any supplements provided to you by a medical professional. If you think you may have high blood pressure, then get checked out rather than take matters into your own hand.
There are a number of things that can be responsible for causing high blood pressure. Alcohol, salt, continued reg use of ibuprofen, birth control pills, as well as other meds, pregnancy, diabetes, obesity, cholesterol, etc, etc. The best course of action is to 1st have your doctor determine the cause so you then can determine what preventative measures to take.
I know that red meat is bad for high blood pressure and to exercise will help alot and to relax too(: