Such “falls” are inevitable in the awareness, so do not be sick at heart. On the other hand, where there are none at the very beginning of this work, you are probably busy thinking rather than watching.
Saving energy has two sides. The first one is related to control, i.e. forced refusal from pursue of some desires in order to save energy for witnessing. The pro of this approach is that the mind knows it since control is its power. It will be happy to exercise control and suppress any desires, including the most innocent ones. In this case, cons naturally step from pros. Suppression of desires creates tension in all the bodies; moreover, the desires will win sooner or later, and you will be submerged into the gulf of unconsciousness, which will be very deep.
The other way to save energy is to refuse from the things you do mechanically, habitually or because these actions are expected by the people around. For instance, one of the hardest aspects experienced at the beginning of the awareness practice is to keep self-observation while talking to someone. As a rule, as soon as the conversation starts, the attention is fully absorbed by the companion’s words and the fact that you must respond. Even if you observe yourself a little, you will make sure that communication takes away lots of your energy and time. Moreover, the considerable part of the time spent by you to talk is just wasted for unconscious chat. You can refuse from such communication quite painlessly for yourself and observe how much the mind likes talking rot.
I think everyone can try and find the actions that have turned into mechanical life habits and are of no use for the mind or heart. You can easily refuse from them. Furthermore, this refusal is temporary, for the period necessary to strengthen the witnessing skill. As soon as awareness gets stable, you can turn back to everything you have refused from. Of course, if you wish to for some unexplainable reasons.