When you repeat the Lord's name you must evince unflinching devotion from the bottom of your heart devotion to God, without love for any other object. You must drive off all worldly thoughts from your mind. Fill the mind with thoughts of God and God alone. You must struggle. You must exert hard. Remain absorbed in him.
If you love Krishna, then love Him alone till the end. Just as you see wood alone in the chair, table, bench, stick, cupboard etc., see the antaratma (the hidden indwelling self) - Krishna alone - in flowers, trees, fruits, tumblers and all objects. This is ananya bhakti (total devotion). This is para (supreme) bhakti.
Just as you remember all the qualities of your son when you think of his name, so too you should remember the qualities of God omnipotence, omniscience etc., when you think of his own name.
When you repeat the mantra, have satvic bhava or shuddha bhava (the pure mental attitude). Bhava comes slowly when the purification process goes on. Even mere mechanical repetition has an effect, a very great effect. The vibration in the mind set up by the repetition purifies the chitta (mind stuff) and brings chitta shuddi (purity of mind).
A beginner should have a japa mala or rosary. Later on he can take recourse to manasika japa (mental repetition). If a man repeats his mantra for six hours daily, his heart will be purified quickly. He can feel the purity. Have great faith in your guru mantra (the mantra learnt from your guru) and keep it a secret.
Constant repetition of the mantra with faith, devotion, purity and one pointed mind awakens the mantra chaitanya (the dynamic consciousness of the mantra latent in the mantra, and bestows mantra siddhi (perfection or psychic power inherent in the mantra) on the aspirant illumination, freedom, peace, eternal bliss and immortality. The repetition of the mantra again and again generates a great spiritual force and momentum and intensifies the spiritual samskaras or impressions.