The basic principle of devotional service is detachment
from material activities and attachment to the transcendental
service of the Lord. (Bhagavad-gita 15.1 purport)
Devotional service means mukti, freedom from material entanglement. (Srimad-Bhagavatam 5.11.4 purport)
By absorbing our senses in activities centred around Krishna, we cultivate attachment to Krishna, which will gradually develop into love, and all our attachment to material activities will be forgotten.
Further reading: Srimad-Bhagavatam 9.4.18–20 or Bhagavad-gita 2.61 purport
pancaratrika-vidhi: devotional service focused on or centred around Deity worship.
bhagavata-vidhi: centred around hearing and chanting the holy name and preaching.
Srila Jiva Gosvami says that although sankirtana is sufficient for the perfection of life, the arcana, or worship of the Deity in the temple, must continue in order that the devotees may stay clean and pure.
(Srimad-Bhagavtam 6.3.25 purport)
The Krishna conscious movement accepts both processes simultaneously and thus enables one to make steady progress on the path of realization of the Supreme Personality of Godhead. (Srimad-Bhagavatam 4.13.3 purport)
Bhagavata-marga will help the pancaratrika-marga, or process, and the pancaratrika process will help the Bhagavata process. Both together is helpful. (Srimad-Bhagavatam Lecture, 13 February 1971)
smartavyah satatam vishnur
vismartavyo na jatucit
sarve vidhi-nisedhah syur
etayor eva kinkarah
Vishnu should always be remembered and never forgotten at any time. All rules and prohibitions mentioned in the sastras should be servants of these two principles.
(Padma Purana)
Life is full of ups and downs. An inexperienced devotee will especially remember Krishna when he's down, but a more experienced devotee also remembers to be thankful to Krsna and to take shelter of Him even when everything seems to be great.
What does the word sadhana mean? According to the Monier-Williams Sanskrit dictionary sadhana is:
The means to achieve an end; carrying out; bringing about; completion
In his books, Srila Prabhupada describes sadhana-bhakti (or the means to achieve pure devotion) in the following ways:
One can compare it to an automobile that needs to be pushed to get it going. Once it starts running, it moves under its own energy.
Therefore in the beginning, the newcomer in spiritual life must render devotional service under the guidance of a bona fide spiritual master according to the regulative principles mentioned in the revealed scriptures.