A waitlist, often called a cancellation list, is a record of clients who would take a time sooner than the one they have, or who could not get in at all, and who want to hear if something frees up. When a client cancels, the business works down the list and offers the slot. Done well, it turns a late cancellation from lost income into a pleased client who got in early.
Keep the list short and specific: name, mobile, service, the days and times they can manage, how much notice they need, and the date they asked. A list of forty names with no detail takes longer to work through than the slot is worth.
In practice
A hair salon keeps a waitlist of 14 clients. On a Wednesday at 7 pm a client cancels a two-hour colour booked for 10 am Thursday, worth $180. The owner checks the list for anyone who can do a Thursday morning at short notice, finds 4, and texts them one at a time in the order they joined, giving each 20 minutes to reply. The second one accepts at 7:35 pm. The slot is filled, and the client who took it has come forward three weeks.
Across a month the salon offers 11 cancelled slots and fills 8 of them, $1,320 of work that would otherwise have been empty time. Texting one person at a time is slower than a group message, but it avoids three people replying yes to the same slot.
Invite people to join when they cannot get the time they want, and take off anyone who has turned down three offers or not replied for a month.