Both Datura and Brugmansia (a close relative) do much better here in Florida if you grow them in pots. We have nematodes in the soil here, and unless you solarize (bake) your soil regularly, they will invade the roots of the plant and cause it to grow stunted, decline and eventually die. Lots of plants here are not affected by the nematodes, but the Dats and Brugs really are!
So that's the first thing. Grow them in BIG pots, and use fresh potting soil to start out with. Don't set the pot directly on the ground, either, as the nematodes can invade the pot through the drain hole after a while.
They grow extremely fast here with the warm, humid weather. So you need to water and feed them like crazy too. Cold nights in winter will slow them down quite a bit, but they'll just take off in spring once the nights are reliably warm.
If you do have one that is failing, try taking a cutting and starting a new plant. It's faster than starting from seeds, too. The cuttings will make roots within a week or two if just kept in a jar of water on a bright windowsill. If there's a big, healthy plant in your neighborhood, ask for a cutting from it. Maybe it's a variety that does particularly well there.
I've honestly never started one from seed, so I don't know how long your seeds will be viable, but no harm in trying.
Btw, we have a whole forum for Brugmansias here at ATP, so come on over and take a look.
http://garden.org/forums/view/...