Question: What should my state of mind be when I do Aikido?
Answer: One's mind should be in a state of calmness. In Buddhism it is referred to as being in a state of equanimity. In swordsmanship it is called the "non-abiding" mind. In sports, it is called, "being in the zone."
This place of equanimity is the highest state of training. One might say it is the goal of training. The non-abiding mind simply means that one's mindset is in a place where it does not stop, but that can be a bit too allegorical. This idea of equanimity really means that one's mindset is to do nothing more than merely observe and to definitely not "react." From this place of observation, one can act accordingly. This place of the non-abiding mind is where one acts appropriately and therefore mindfully. To react is to become aroused and thus act mindlessly.
By reacting we generally choose poorly. All of us have experienced this at one time or another. For instance, we have all gone to the market on an empty stomach and end up making bad choices or getting our feelings hurt and lashing out at other people. The objective of training is to give us options. By being in a place of mindfulness we can better chose the appropriate action.
Thus, we can see that mindfulness is the goal of training.