We can borrow some ideas from military medicine. I first got interested in this when I found out that banks, brokerages, telcos and insurance companies were using it to retain customers.
Triage a system the military uses to judge how serious injuries are and to prioritize treatments. Companies adapted triage to use in their data mining systems to highlight customers who seemed to be taking actions to leave the company. They determined some events and conditions that would take place that might indicate the customer was not going to stay. When they spot this they will approach the customer with special offers or discounts to retain them.
How could this be applied to our retention efforts?
A very simple idea would be to call a student when he/she missed class. Just say Hello and show your concern if the student is ill or whatever. This will help the student feel closer.
SIGNALS A STUDENT MAY QUIT
If two classes in a row are missed this could be the first step towards dropping out. You could call the student to just see how things are going and how does the student feel about the lessons. Of course, when there are three absences in a row then this may be a clearer indication of the student dropping out and the teacher should check to see if something is happening.
A student's "suggestions" are sometimes complaints. All the time the teacher should be listening to the students to see if the students feel the lessons are meeting their needs and if they are satisfied. Sometimes the lessons are perfect for the students but the students don't know it. In this case, the teacher may need to explain more, do more salesmanship for the reason of the approach, materials, etc.
HELPING THE STUDENTS SEE HOW THEY ARE PROGRESSING
Sometimes it is easy for students to not "feel" they are progressing. If they feel they are not making progress it makes it much easier for them to decide to quit. If they feel they are making progress they are more inclined to solve any external problems that may hinder their attendance. Language acquisition is so incremental that many students don't notice it. It's like my children. I can't see them growing day-by-day but I'm pretty sure they are.
I think it is interesting how Cambridge Interchange 3 (formerly New Interchange) has added a very short two-page "Self-Assessment" after every two units. This assessment has six questions on the things that the student had learned in the previous two units.
For example, Book Two for Unit 3-4 asks, How well can you do these things? Very well, OK, A little: Ask and answer questions about prices. Give opinions using adjectives. Talk about preferences and make comparisons with adjectives. Etc.
The student makes a self-assessment on these points. The following these six questions are exercises on each of these points. The student can go through the exercises and see how well he/she does and what, if anything, needs further explanation. This way students can see exactly what they are learning and exactly how they are making progress.
OTHER THINGS TO MEET STUDENTS' NEEDS
Sometimes the students have a legitimate need. They want more variety, a couple games thrown in to spice things up and make them a bit more fun, more or less homework, etc.
There may be domestic or professional pressures making it difficult for the student to attend. Perhaps a change in the schedule or greater flexibility in the schedule can help.
 Dictionary definition: "Triage is a system employed in military medicine for the evaluation and classification of casualties. Its primary purpose is to categorize the wounded for treatment and further evacuation where necessary. In effect, triage consists of two main elements. First, the immediate sorting of casualties according to the nature and seriousness of their wounds and the likelihood of their ability to survive them, with or without intervention and with consideration of the available resources. Second, the establishment of priorities for treatment and subsequent evacuation, in order that medical care is provided for the greatest benefit of the largest number of casualties."