Dharana means “maintaining a proper concentration”. Proper concentration means keeping the indriyas on God. In other words, this is a real manifestation of our aspiration to God, to Mergence with Him.
But God in the aspect of the Creator or the Holy Spirit is inaccessible for direct perception at this stage of apprenticeship.
Our loving thirst for God can be partially quenched by working with an Image of a Divine Teacher, for example, Jesus Christ, Babaji, or Sathya Sai Baba — the One Whose form from His past Incarnation is familiar to us.
If we hold such an Image in anahata on the background of the emotion of the most intense love that we are capable of, we gradually enter the state when it is not I who look at the world from anahata but He. This denotes the Yidam (this is what this Image is called) becoming alive; we are partially merged with Him. After that we may live in Unity with Him in anahata; having moved the concentration of the consciousness to the chakras located in the head, we can address Him in anahata as an Advisor and a Teacher.
This is not an illusion but the real Divine Teacher entering into His Image created by us. He may also become an Instructor in our meditative trainings. He will lead His devoted and loving disciples through Himself — into the Abode of the Creator.
“If you can visualize the Image of the Teacher in your consciousness with the most complete clarity, you can transfer your consciousness into His, and thus act through His Power, as it were. But for this, you must visualize the Image of the Teacher with utmost precision, to the minutest detail, so that the Image does not flicker, suffer distortion or change Its outlines, as it frequently happens. But if following the exercise of concentration one succeeds in invoking the steady Image of the Teacher, through this one may gain the greatest benefit for oneself, for those around one, and for the work.” .
“You may be asked how the entrance upon the path of Service is defined. Certainly, the first sign will be renunciation of the past and total aspiring towards the future. The second sign will be realization of the Teacher within the heart not because it is one’s “duty”, but because it is impossible otherwise. The third sign will be rejection of fear, for the one who is armed by the Lord is invulnerable. The fourth will be non-condemnation, because the one who strives into the future has no time to occupy oneself with the refuse of yesterday. The fifth will be the filling of the entire time with labor for the future. The sixth will be the joy of Service and completely offering oneself for the good of the world. The seventh will be spiritual aspiration towards the far-off worlds as a predestined path. According to these signs you will discern warriors that are ready and manifested for Service. They will understand where to raise the sword for the Lord, and their words will be from the heart.” 
If work with Yidam does not bring immediate results, one may benefit from practicing visualization. One may practice creating images that help develop the chakras or visualize blissful pictures of communion with living nature, etc. But only those images, which are filled with exultation of happiness, harmony, joy, subtlety, and bliss, will make for one’s correct spiritual development. Corresponding types of paintings, musical compositions and art photography, etc. may also serve as an aid.