It's pretty simple: just take each spent bloom and trim the stem down to the next node where you see two offshoots. Within a few weeks, you will once again have fresh new stems with blooms ready to open. This technique is called 'deadheading' and many a perennial and annual will respond in a similar fashion.
This particular salvia is a perennial, hardy to zone 5. And it grows like nobody's business, multiplying in size year after year. In fact, this fall will be the first year that I divide it after having had it in the same galvanized tub for three years and I fully expect to have to take a hacksaw to it to break it apart. The good news is, I'll have at least five new plants as a result of that effort. Maybe I'll share.
Bookshelf: Container Gardening Books