If they are annuals they only live for one season and reproduce by seed. Save the seeds.

No, I'm trying to avoid 'starting over". It takes usually takes brugmansias more than one year to start flowering. And I've never heard of anyone growing patchouli or mandevillas from seed....

I keep my mandevilla in its pot in a slightly cooler than average room in front of a window. It gets a bit spindly, but comes back when I cut it back some and put it outside for the summer.

I've heard of brugmansias being overwintered by taking cuttings when there isn't room for the entire plant, also keeping it in a bright area.

I have seen one person who lets his Datura go to seed (he grows it in a giant tractor tire as a planter, so I imagine it warms up fairly quickly in spring) and lets the seed grow as they want in spring. He said he used to start the seed indoors every year, but this is less trouble. The scent was heavenly.

I don't know anything about patchouli.

To address Wayne's comment back in September. Botanically speaking all these plants are perennials, not annuals. Indeed some are woody shrubs. So although they are thrown out after one season in cold climates they live for years in their natural habitats and can be made to do so in less favourable ones.