I don't think "a" prayer can help your physical health, but I do think the act of prayer (or meditation), done consistently, can. I'm sure there's a study out there somewhere conducted by some mind-body scientist that shows (or attempts to show) the connection, but to me, it just makes sense. Here's why:
Prayer is about spirituality; about reaching inside yourself to get outside of yourself, thinking about and feeling bigger, transcendent things. It's a way of calming oneself and so, therefore, it seems to me that this has got to be good for you in terms of positive body chemistry. Now, if you pray (or meditate) a lot and purposefully, but then you eat lots of red meat and ice cream equally consistently, my thinking is that prayer aint gonna do a whole lot for you physically. BUT, if you do the prayer/meditation thing and do other things right, my sense is that you'll not only have a long life but one that is generally free of unnecessary suffering. Note, I didn't say free from suffering...just unnecessary suffering. After all, life is suffering... and your amount of suffering is largely a matter of degree and can be somewhat controlled by you... right?
My prayer practice is somewhat inconsistent and I'm still working on it... which is why I guess I refer to it...borrowing again from Zen... as practice. I try to wake up before dawn five or six days a week. My practice consists of either (1) handwriting three pages of stream-of-consciousness (an idea from "The Artist's Way" by Julia Cameron); (2) reciting the morning blessings and Shema from my Jewish prayerbook; or (3) meditating for 20-30 minutes. On Saturday mornings I don't wake up with the dawn, but I attend the Sabbath service at my synagogue, which is extremely soulful and lovely. On Sundays, taking a cue from Lyle Lovett, I usually sleep-in with my wonderful wifey. Every other Friday morning I go to a yoga class, which is just amazing and very spiritual. There's really no direct payoff from my practice, but I think overall it provides me with peace of mind. Whether it'll help me live longer, I don't know and I don't really care. I do know that I'm afraid of death. I hope that when my time comes, it's from old-age and it's fast.