What things should a human soul attach itself to?

Happy is the pure soul who does not attach himself to transient conditions and comforts, but rather seeks to attach himself to the purity, nobility and splendor of the world which endures. 
(‘Abdu’l-Baha, Baha'i Scriptures; Star of the West, vol. IV, No. 16, December 31, 1913)

Meaning of the term “soul” in various kingdoms of creation -- 'Abdu'l-Baha explains

As to the question concerning the soul, know thou, verily, that "soul" is a term applied to numerous realities, according to the exigencies of the following relations in regard to development in the world of existence:

(1) In the mineral kingdom, soul is called "latent force," silently working for the disintegration of the substance of the mineral.

(2) In the vegetable kingdom it is called “virtue augmentative," or the power of growth, which attracts and absorbs the delicate materials of inorganic substance found in the mineral kingdom of matter, and transforms them into the condition of growth. Thus the inorganic substance found in the mineral kingdom becomes growing vegetable life through the effect of the word of God. This vegetable soul, i. e. "virtue augmentative," or power of growth, is a quality which is produced by the admixture of elements, and appears in accidental organisms, of which contingency is an essential attribute.

Prayer is the “spiritual nourishment” for the human soul

It is the soul of man that has first to be fed. And this spiritual nourishment prayer can best provide. Laws and institutions, as viewed by Bahá'u'lláh, can become really effective only when our inner spiritual life has been perfected and transformed. Otherwise religion will degenerate into a mere organization, and become a dead thing. 
(From a letter dated 8 December 1935 written on behalf of Shoghi Effendi to an individual believer, published in "Bahá'í News" 102 (August 1936), p. 3; The Compilation of Compilations, Vol. II, The Importance of Prayer, Meditation and the Devotional Attitude)

The “body is dependent upon the soul”

Furthermore, many say, `We have no need of divine Manifestations; we ourselves have direct communication with God.' They do not know that the divine Manifestations are the bright rays of the Sun of Truth and a means of educating the realities of man. Therefore, he who rejects the bounty of the Sun of Truth and thinks himself not in need of it is like the one who says he is not in need of God and rejects both God and reality, in spite of the fact that all creation is receiving incessant bounty from God and is dependent on Him, as the body is dependent upon the soul. 
(Words of ‘Abdu’l-Baha, New York, November 26, 1912, Mahmud’s Diary)

“…greed and envy becloudeth the radiance of the soul even as the clouds obstruct the light of the sun.”

O children of understanding! If the eyelid, however delicate, can deprive man’s outer eye from beholding the world and all that is therein, consider then what would be wrought if the veil of covetousness were to descend upon his inner eye. Say: O people! The darkness of greed and envy becloudeth the radiance of the soul even as the clouds obstruct the light of the sun. Should anyone hearken unto this utterance with a discerning ear, he will unfurl the wings of detachment and soar effortlessly in the atmosphere of true understanding. 
(Baha’u’llah, ‘The Tabernacle of Unity, Bahá’u’lláh’s Responses to Mánikchí Sáhib and Other Writings’)

How a human soul can become nearer to God – ‘Abdu’l-Baha explains

Nearness to God is dependent upon purity of the heart and exhilaration of the spirit through the glad tidings of the Kingdom. Consider how a pure, well-polished mirror fully reflects the effulgence of the sun, no matter how distant the sun may be. As soon as the mirror is cleaned and purified, the sun will manifest itself. The more pure and sanctified the heart of man becomes, the nearer it draws to God, and the light of the Sun of Reality is revealed within it. This light sets hearts aglow with the fire of the love of God, opens in them the doors of knowledge and unseals the divine mysteries so that spiritual discoveries are made possible. All the Prophets have drawn near to God through severance. We must emulate those Holy Souls and renounce our own wishes and desires. We must purify ourselves from the mire and soil of earthly contact until our hearts become as mirrors in clearness and the light of the most great guidance reveals itself in them.

Bahá’u’lláh proclaims in the Hidden Words that God inspires His servants and is revealed through them. He says, “Thy heart is My home; sanctify it for My descent. Thy spirit is My place of revelation; cleanse it for My manifestation.” Therefore, we learn that nearness to God is possible through devotion to Him, through entrance into the Kingdom and service to humanity; it is attained by unity with mankind and through loving-kindness to all; it is dependent upon investigation of truth, acquisition of praiseworthy virtues, service in the cause of universal peace and personal sanctification. In a word, nearness to God necessitates sacrifice of self, severance and the giving up of all to Him. Nearness is likeness.

Behold how the sun shines upon all creation, but only surfaces that are pure and polished can reflect its glory and light. 
(‘Abdu’l-Baha, from a talk, 26 May 1912, New York, ‘The Promulgation of the Universal Peace’) (To see the entire talk please visit Talks of ‘Abdu’l-Baha)