If the heart wanders or is distracted, bring it back to the point quite gently -- and even if you do nothing during the whole of your prayer but bring your heart back, though it went away every time you brought it back, your time would be very well employed.
Mindfulness is an ancient form of meditation in which one pays attention to the present moment and all that's unfolding in that moment, both within and around one. It's known also as conscious living because the person practicing it is forming an aware and intimate relationship with each moment.
When practicing mindful meditation we aren't striving to do anything, we aren't grasping, struggling, thinking, expecting, or wanting but simply letting whatever is there be there and paying attention to it in a non-judgmental way. We come to terms with reality as it is, bringing all our awareness to it, breathing with it, attending it.