Want a really effective diuretic? Look at the green stuff under your feet.
Hair-cap moss (Polytrichum juniperinum) is a very common moss, up here, and while I haven't gotten around to taking pics of it yet, I will, this summer, never fear.
It's a pretty little moss, deep dark green, with brownish-red sporeheads that look like strange small tall hats on long stalks that rise above the moss proper. Cool plant. Lemmesee if I can find a pic - yes, lots, with a very nice one here.
King's says it's a strong diuretic, especially in dropsies - dropsy being edema, and in old books that often means the edema that comes with heart failure.
See? Petersen confirms: "especially cardiac dropsy".
The Eclectics also say that it's good for irritation of the bladder, and for urinary stones.
Cook says you shouldn't overdo it, though.
And J. King says that it doesn't always work. If what Cook says is true ("relaxes the kidneys to produce copious amounts of water") then it wouldn't work on those with a "cool" liver (as per Michael Moore's energetics (.pdf here)) - those folks already have very watery urine.