I've always had low blood pressure and have never had a problem. I've gotten a little dizzy on occasions (when I've jumped up too fast from sitting) but I've never fainted. When I was at the end of my pregnancy with DS my midwife took my blood pressure and was a bit shocked that it was 75/40. I felt fine though! Last time I had it checked it was 84/60.

I guess the only thing I can suggest is being careful going from sitting to standing.