The most common cause of the tail belt loosening is the tail boom slipping forward in the tail block. To prevent the slipping you can wrap a small piece of packing tape around the boom where it is gripped by the tail block. This tape increases the OD of the boom to allow the block to get a better grip.
Alternatively, you can put an extra tail servo mount on the boom right up against the block, the mount will also keep the boom from sliding back.
Contributed by Glucoseboy