1) Reach back out to past clients that you have cleaned for 3 months ago, 6 months ago, and a year ago. Check to see how things are, and remind the 12-month folks that it is recommended to have your carpet cleaned every 12-18 months. Approach it is, a lot of your customers prefer to be reminded so that is just something you do to help stay on top of it.
2) Hit the pavement. Pass out business cards. Search for other small businesses like HVAC, plumbers, dentists, doctor's offices, and other small commercial buildings. Half the time you can see what it looks like inside from Google and get an idea if they even need your services. Put a list together and go visit, ask to offer an estimate/evaluate their, ask who makes decisions on their floor care, or simply give them a card and other promotional materials you might have. You can offer to do a demo as well.
3) Post to Google, Facebook, and Instagram. Each of them gives you the ability to post photos and videos of your work. Do it regularly.
4) Reach back out to past customers and ask for reviews.