Here are four common yoga concentration techniques for the development of dharana that you can experiment with:
Trataka

Trataka (which means steady gaze) is a technique whereby the eyes are fixed upon a certain external point such as the horizon of the sea, or the flame of a candle.
Trataka1

* Take a comfortable seated position outdoors, where objects at various distances between you and the horizon can be viewed.
* Pick at least 4 objects (roughly spaced equal distance), the first being near the feet and the furthest at the horizon.
* Beginning at the nearest object, concentrate upon it for 60 seconds.
* Move you attention every 60-seconds to each successively further object until the final one at the horizon has been reached.
* After focusing upon the last point for 60 seconds, reverse object by object (again concentrating 60 seconds on each) back to the first point.

Trataka2

* Place a candle upon a small table, whereby the flame is at eye-level.
* Sit a distance of between 6 to 10 feet away and place your concentration upon the flame.
* Do not to blink and try to hold you concentration for between 30 and 60 seconds.
* Close you eyes for 30 to 60 seconds and try to reproduce the image of the flame in your ‘mind’s eye’.
* Repeat this process 3 to 6 times in one sitting.

Nasarga Drishthi

* Take a comfortable sitting position and gaze open-eyed at the tip of the nose.
* Hold this concentration until you feel tension in the eyes, then
* Close your eyelids, but imagine that you are still gazing at the tip of the nose.
* Hold this visualization for 15-20 seconds and then re-open the eyes.
* Repeat this process several times.

Bhrumadhya Drishthi

* Again, take a comfortable sitting position and turn the gaze to the base of the nose, creating a ‘cross-eyed’ position.
* Raise the eyes slightly, so that this cross-eyed gaze now falls directly between the eyebrows, then…
* Close your eyelids and hold this point of concentration ‘inwardly’.
* Periodically open your eyes to see if this point is being maintained, then close them once again.
* Hold this concentration for several minutes