Concentration is a byproduct of meditation. Meditation is de-concentration.
Do you have the notion that meditation is concentration? It is not so. In fact it is the opposite of concentration. We need concentration for doing work but for meditation you do not need concentration. Meditation results in alertness and focus of mind.
How to increase concentration?
Learn pranayam. Doing Sudarshan Kriya and meditation regularly increases your energy levels. When your energy levels change, your mind becomes more calm and concentrated