Cuttings:
Salvia divinorum propagates naturally and easily by rooting from cuttings. They will root in plain tap water (without the need for rooting hormone powders or the like) normally within two or three weeks. 

The best time to do this is probably in the spring with the daylight extending, but in practice you may tend to do it more in the autumn, when mature plants should be cut back anyway and it would otherwise mean not utilising the cuttings.  You can take cuttings whenever you like though, and I generally recommend getting as many going as soon as you can, especially for the insurance of having more than one plant. The growth rate is dependent on the temperature and the amount of light, for example, a cutting taken in mid-winter should still work but could take 4-6 weeks to root rather than 2-3 weeks. Artificial light can be used in the winter, see the section on lighting.

Use a clean sharp knife to take a cutting ideally about 20-30cm in length (but anything above 10cm will probably be okay). You'll notice that the stem has distinct sections. Since the remaining stem on the live plant will die back to the next section, it's a good idea to take cuttings near to a join between two sections (to minimise such wastage), that is, from about 2cm above an intersection.

It's best to take cuttings of a reasonable size.  Good thick stems means more biomass and ensures a reserve for the plant to draw upon while setting it's roots so improving the chances of getting quickly established vigorous plants. However, younger shoots can be used too, and if you are trimming the plant or 'pinching' it, you may as well have a go with whatever cuttings you take.

Rooting the cuttings in separate containers will isolate good from bad (in case one fails and starts to rot) and improve the over all chances of success.  

Simply leave them in their containers (I use glass jars) near a window but avoiding direct sunlight. Make sure that there is enough water in the container, about 6-10cm depth of water for an average length cutting. 

Cuttings are ready for potting when you have a good few roots 2 - 4 cm long.

 

Cutting plants back: Plants should be cut back in the autumn.  Their growth rate will slow in the winter and if you don't do this then they may not be able to sustain themselves.  I've had one large plant die after not being cut back (admittedly it was also in a particularly shady corner).   

However, as suggested elsewhere, if you are relating these notes to your own circumstances, which may be different, then this may be less of a factor.  Here in Scotland, the winter nights are about twice as long as the days (- and the days can be pretty dark too!). Also, remember that plants can grow under artificial light, check out the section on lighting back from here.