Lama Tsong Khapa:
The nature of mental stabilization is a virtuous mind abiding single-pointedly on its object, without mental wandering.
They who wish to protect the trainings 
Protect the mind after focusing it strongly.
Without protecting this mind
It is impossible to protect the trainings.
Showing in Brief the Necessity of Protecting One’s Mind
They who wish to protect the trainings of generosity and the other perfections from degeneration must strongly focus their mind on the trainings, and then protect the mind from wandering off to mistaken objects.
If one does not protect the mind from wandering off, then it becomes impossible to protect the trainings.
Ven Geshe Doga:
How to Meditate: A crazed elephant can be tamed with a rope, hook and pillar. It is tied to the pillar with the rope, and directed with the hook. Similarly, our crazed mind is subdued with the rope of mindfulness, the hook or introspection, and the pillar of the virtuous meditation object.
First the mind needs to be tied to the pillar of the meditation object with the rope of mindfulness, and then we use the hook of mental introspection to direct the mind back to the virtuous object if it has strayed, or adjust our focus when we find the mind has become unclear.
Through this approach, we will not experience mental wandering and distraction, and all other virtuous practices will arise naturally.