When meditation stales,
Change methods quickly.
For those who follow Tao, there is no such thing as just one meditation that you practice for the rest of your life. All of Tao changes and flows, so too should meditation. It is not a static discipline but rather a progressive means of spiritual living. Beginners have their meditation, advanced students have theirs. Simple people have simple meditations, complicated people must have meditations that engage them fully.
No matter what kind of person you are, there are times when you will exhaust all the potential of a certain contemplative method. After all, a method is only an arbitrary structure, whereas the subconscious that you are trying to master is infinite, changeable, elusive. Therefore, when a method is exhausted, you have to change to another one. Sometimes, it is enough to switch back and forth between methods; at other times, you will need to go to a higher stage of meditation.
As long as you feel restless, it is a sign that you have not yet become fully mature on the spiritual path. The ultimate levels of meditation deal with a complete stillness of the mind. In this state, one feels nothing, thinks of nothing, worries about nothing. When meditation becomes stale, there is a preoccupation that will prevent you from attaining this stillness. That is why you change, until the day when restlessness naturally recedes and stillness is all that remains.