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.
We need to sit still, let our emptiness remain empty, and wait patiently. If we fill the foreground with busy questions, reasons or proposed actions, we will miss the God who is the silent, yet ever present horizon of the world.