His Divine Grace A. C. Bhaktivedanta Swami  Prabhupada
Just as a devoted wife becomes afflicted at the passing away  of her husband, when a spiritual master passes away, the disciple becomes  similarly bereaved. (Purport to Bhäg. 4.28.49)
 
The spiritual master, äcärya, is always situated in the  spiritual status of life. Birth, death, old age and disease do not affect  him. According to the Haribhakti- viläsa, therefore, after the disappearance  of an äcärya, his body is never burnt to ashes, for it is a spiritual  body. The spiritual body is always unaffected by material conditions.  (Purport to Bhäg. 10.4.20)
SB 7.7.30-31       What Prahlada  Learned in the Womb     The words guru-susrusaya  mean that one should personally serve the spiritual master by giving him  bodily comforts, helping him in bathing, dressing, sleeping, eating and so  on. This is called guru-susrusanam. A disciple should serve the spiritual  master as a menial servant, and whatever he has in his possession should  be dedicated to the spiritual master. pranair arthair dhiya vaca. Everyone  has his life, his wealth, his intelligence and his words, and all of them  should be offered to the Supreme Personality of Godhead through the via  medium of the spiritual master. Everything should be offered to the spiritual  master as a matter of duty, but the offering should be made to the spiritual  master with heart and soul, not artificially to gain material prestige. This  offering is called arpana. Moreover, one should live among devotees, saintly  persons, to learn the etiquette and proper behavior of devotional service.  Srila Visvanatha Cakravarti Thakura remarks in this connection that  whatever is offered to the spiritual master should be offered with love  and affection, not for material adoration.
Whenever an äcärya comes, following the superior orders of the  Supreme Personality of Godhead or his representative, he establishes the  principles of religion. Unfortunately, when the äcärya disappears, rogues and  non-devotees take advantage and immediately begin to introduce  unauthorized principles in the name of so-called svämés, yogés,  philanthropists, welfare workers, and so on. The äcärya, the  authorized representative of the Supreme Lord, establishes these  principles, but when he disappears, things once again become disordered. The  perfect disciples of the äcärya try to relieve the situation by sincerely  following the instructions of the spiritual master. (Purport to Bhäg.  4.28.48) ·