We have two courses with lower than normal enrollment. We advertised for them the same way as was done in previous years. Neither course was offered last year. What else can we do to recruit? Classes start next week.
If you have academic advisors or advisors for the major, you could reach out the them. Let them know that these classes fulfill requirements X, Y and Z.
Another strategy is to relax on the pre-requisites (very situation dependent). Maybe taking 1 quarter of chemistry/math/composition/etc is enough rather than the full series?
If your students are in person, you could try the really old-school approach and make flyers to hang in the hallways of your building.
"Need another [Basketweaving] class to graduate? Back by popular demand after a year of not being offered, it's [Baskets 202]! Do [exciting thing 1], [exciting thing 2], and gain [awesome skill Y]! Sign up NOW! Got questions - email Dr. Katz"
You can directly contact students who did well in one of your courses recently. You can posterize residences, the meal hall, the offices of cognate departments.
Longer-term,you can make sure the academic advisors 8n the registrar's office know about your course.
+1 to posters (if in person). Get a student to hang them - they know where the other students actually go.
For the future, I also recommend previewing upcoming courses around registration time. In all of my intro courses I give a quick preview of what courses the dept. will be offering in the next semester. I talk about what requirements those courses fulfill, how easy it is to get a minor/major, and the benefits of a minor/major in my field.