Cutting, trimming. When the plants get big. I generally cut
just above a node. The stem above the node is going to dry up
anyway, you don't want the hollow stem to fill with water and
rot. Any place you cut, new branches will sprout.
Harvesting is a good time to start new slips. Say you cut off
a four foot section of stem. Break off the top eight to twelve
inches or so, pinch off all but the small leaves near the top,
and drop it into a glass of water. If there are four or five such
slips in one glass, they seem to stimulate each others root
growth. When the rootlets bud out, you can pot them. I never
use rooting compound anymore, though there is no reason not to.
You can also start the mid-sections of stem--it doesn't have to
be the top. A couple of such exponential cycles are you'll be
well-endowed.
If you are in an arid climate, or have access to such, drying
the leaves is easy. Salvinorin is remarkably stable.
However, dried leaf is not quite like fresh. The fresh leaves
you can ingest. Dried, smoke. Somewhat analogous to dmt/ayahuasca.
(discounting the harmine effects...).
good luck.
I use two methods of harvesting, picking and cutting.
Picking is just that: carefully pinching off a percentage
of the larger leaves. They grow back double, though smaller.
When pinching, pinch at the base but don't take the meristematic
tissue on the sides--that's where the new leaves come from.