Balanced practice - a balance between mindfulness and wisdom and clear comprehension, that needs to be balanced because if we only have awareness we may not be aware of the laws governing our existence and how an action in this moment will affect a future moment. So it's not just that we just become aware of the present moment and allow anything to happen. The balance between mindfulness and clear comprehension is very important so that we have mindfulness and wisdom.
The balance of the Unselfish Emotions is also very important. Compassion, Lovingkindness, Sympathetic Joy, Equanimity. Within Compassion and Lovingkindness, Lovingkindness helps to balance Compassion because we understand the potential for beings, for the ending of Dukkha. Equanimity also helps to balance Compassion in that we see that it is not our responsibility to end the Dukkha of all beings. So we understand what we can and what we cannot control. Sympathetic Joy helps to balance Compassion and Lovingkindness by focusing on the beneficial qualities of ourselves and others. Compassion, Equanimity and Lovingkindness help to balance Sympathetic Joy.
So there is a balance within the Brahma Viharas.
There is also a balance between letting go of thought, the awareness and investigation techniques on the moment-to-moment-level and the balance with Yoniso Manasikara; that is learning how to reflect wisely - the reflection side. If you are reflecting all the time then you may tire your mind out. So you also have to have the balance of letting go of thought and just observing the nature of thought to balance this. If you are only concentrating the mind of being mindful,, then you aren't developing more of the Right View that prevents the hindrances from arising. So this is a quick overview as there are many aspects of a balanced practice.