On the highest level, Krishna is the symbol of the Immortal Being and Radha, his Shakti; an inseparable, timeless and endless bond. This path of worship is also the integration of the Advaita and Dvaita Principles of Indian Spiritualism. The Bhakti Marg by which one loses ones being and existence into that of the Supreme Entity; Samadhi through Pure Love.
The meditation mantra chanted by some of devotee is just the name of Radha, like this - Radhe Radhe; राधे राधे.
The vocal chants are a continuous repetition; with a deep longing, a kind of yerning from the bottom of the heart to become one with Radha and merge ones existence with her. Then there are those who practice mental chants, the same way and are totally lost in the meditation.
There various other Mantras and Tantra of worshipping Radha; however the one given above seems to be direct and most effective.