You need to take axillary growth (not the really thick main stem) of plants that are growing rapidly. Take a 6 - 8 inch tall plant, lay it on its side with the stem in contact with soil. This will cause the stem to root and the axillary growth to put out roots of its own. As long as you leave some nodes in contact with the soil, they will keep generating new shoots. It will take two years to get a big stock plant with many stems.
Most any of the sages that tend to form single, thick stems with little axial growth would benefit from this technique. It is not necessary for those that easily generate axial growth when pinched back like S. mexicana and S. gesneriflora..