Apparently, Olive leaf is an ancient remedy whose first recorded use was in 1000 BC. Exact cause of the lowering of BP isn’t known but it does seem to have effect on most people. I haven’t tried it, but apparently you can buy Olive Leaf Extract at health food stores.
http://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&term=%22Phytotherapy%20research%20%3A%20PTR%22[Jour]%20AND%202008[pdat]%20AND%20Perrinjaquet-Moccetti[author]
http://en.wikipedia.org/wiki/Olive_leaf