I've had BP that was on the lowish end of normal for most of my life. When I started eating paleo/primal, it dropped signifiantly lower (to 90/60 or lower) to the point it started causing problems like lightheadedness and dizzyness when standing up and bending down, and my fatigue also got significantly worse, which I think is connected with the lower BP. So, I began my epic quest to understand how to raise my BP, and here is what I've learned/experimented with:
1) Salt rarely increases BP, so you could try adding more salt to your diet, but likely this won't make a difference. On a similar note, being dehydrated will probably cause BP to go lower, and drinking more liquids will raise BP a little, but this is a temporary fix (unless you are chronically dehydrated).
2) Having a little too much/not enough blood can cause higher/lower BP. My husband was told by his doctor that he has 1/2 a liter too much blood, which is why his BP tends to be on the higher end of normal. From what I've read, this is not a factor that can be changed.
3) Vasodilation (when blood vessels become bigger) causes lower BP, and vasorestriction (when blood vessels shrink) causes higher BP. If BP is too low, it's better to focus on reducing/eliminations things that cause vasodilation (such as alcohol) and try to increase things that cause vasorestriction. This will likely provide temporary relief as blood vessels are constantly expanding or restricting depending on many factors.
4) Licorice can raise BP. Not the fake anise-flavored candy that's called 'licorice', but actual licorice, which is hard, black, and shiny, and does not contain any other ingredients. I was able to find this very easily in Italy, but in the U.S. I've only seen it available on Amazon, and it's pretty expensive. Alternatively, you could buy licorice root, and boil it for 20 minutes to make a tea, and consume that a few times a day. I took licorice daily over the summer, and there was a good increase in my BP (up to 110/70, on average) after a few weeks.
5) HISTAMINE INTOLERANCE. One of the major symptoms of this is low blood pressure. Other symptoms include digestive upset (like diarrhea, constipation, and general IBS-type symptoms, fatigue, and headaches after consuming wine, beer, or other fermented foods and drinks. I'm suspicious that this is ultimately the cause of my low BP, because when I went paleo, I dramatically increased my intake of histamine containing/releasing foods and drinks (like kombucha and ferments and certain veggies and cured meats).