The most acceptable prayer is the one offered with the utmost spirituality and radiance; its prolongation hath not been and is not beloved by God. The more detached and the purer the prayer, the more acceptable is it in the presence of God.
(The Báb, Selections from the Writings of the Báb, p. 78) 
The reason why privacy hath been enjoined in moments of devotion is this, that thou mayest give thy best attention to the remembrance of God, that thy heart may at all times be animated with His Spirit, and not be shut out as by a veil from thy Best Beloved. Let not thy tongue pay lip service in praise of God while thy heart be not attuned to the exalted Summit of Glory, and the Focal Point of communion. Thus if haply thou dost live in the Day of Resurrection, the mirror of thy heart will be set towards Him Who is the Day-Star of Truth; and no sooner will His light shine forth than the splendour thereof shall forthwith be reflected in thy heart. For He is the Source of all goodness, and unto Him revert all things. But if He appeareth while thou hast turned unto thyself in meditation, this shall not profit thee, unless thou shalt mention His Name by words He hath revealed. For in the forthcoming Revelation it is He Who is the Remembrance of God, whereas the devotions which thou art offering at present have been prescribed by the Point of the Bayán, while He Who will shine resplendent in the Day of Resurrection is the Revelation of the inner reality enshrined in the Point of the Bayán—a Revelation more potent, immeasurably more potent, than the one which hath preceded it.
(The Báb, Selections from the Writings of the Báb, pp. 93–94) 
As for devotions other than obligatory prayer, if these be chanted jointly and with a pleasant and affecting melody, this would be most acceptable.
(‘Abdu’l-Bahá, from a Tablet—translated from the Persian) 
The simplicity characterizing the offering of Bahá’í prayers, whether obligatory or otherwise, should be maintained. Rigidity and rituals should be strictly avoided.
(In the handwriting of Shoghi Effendi, appended to a letter dated 30 October 1936 written on his behalf to an individual believer) 
‘Abdu’l-Bahá once said: “The worshipper must pray with a detached spirit, unconditional surrender of the will, concentrated attention and spiritual fervour…. Automatic, formal prayers which do not touch the core of the heart are of no avail.”
(From a letter dated 19 October 1925 written on behalf of Shoghi Effendi to an individual believer) 
As regards the chanting of Tablets in the Temple, Shoghi Effendi wishes in this connection to urge the friends to avoid all forms of rigidity and uniformity in matters of worship. There is no objection to the recital or chanting of prayers in the Oriental language, but there is also no obligation whatever of adopting such a form of prayer at any devotional service in the auditorium of the Temple. It should neither be required nor prohibited. The important thing that should always be borne in mind is that with the exception of certain specific obligatory prayers Bahá’u’lláh has given us no strict or special rulings in matters of worship whether in the Temple or elsewhere. Prayer is essentially a communion between man and God, and as such transcends all ritualistic forms and formulae.
(From a letter dated 15 June 1935 written on behalf of Shoghi Effendi to an individual believer) 
Daily prayers—with the exception of the specific obligatory prayers such as the “Namáz”2—can be recited in any fashion or manner which the believer chooses. Uniformity in the case of such prayers should under no circumstances be imposed upon the friends. The worshipper should be left entirely free to pray as he wishes.
(From a letter dated 6 July 1935 written on behalf of Shoghi Effendi to an individual believer) 
With regard to your spiritual experiences, the Guardian has been very interested to share them. He would, however, urge you to always use and read, during your hours of meditation and prayer, the words revealed by Bahá’u’lláh and the Master.
(From a letter dated 6 December 1935 written on behalf of Shoghi Effendi to an individual believer) 
The true worshipper, while praying, should endeavour not so much to ask God to fulfil his wishes and desires, but rather to adjust these and make them conform to the Divine Will. Only through such an attitude can one derive that feeling of inner peace and contentment which the power of prayer alone can confer.
(From a letter dated 26 October 1938 written on behalf of Shoghi Effendi to an individual believer) 
He feels more emphasis should be laid on the importance and power of prayer, including the use of The Greatest Name, but not over-emphasizing it. It is the spirit behind the words which is really important.
(From a letter dated 16 March 1946 written on behalf of Shoghi Effendi to an individual believer) 
The daily prayers are to be said each one for himself, aloud or silent makes no difference. There is no congregational prayer except that for the dead. We read healing and other prayers in our meetings, but the daily prayer is a personal obligation, so someone else reading it is not quite the same thing as saying it for yourself.
(From a letter dated 31 January 1949 written on behalf of Shoghi Effendi to an individual believer) 
The powers latent in prayer are manifested when it is motivated by the love of God, beyond any fear or favour, and free from ostentation and superstition. It is to be expressed with a sincere and pure heart conducive to contemplation and meditation so that the rational faculty can be illumined by its effects. Such prayer will transcend the limitation of words and go well beyond mere sounds. The sweetness of its melodies must gladden and uplift the heart and reinforce the penetrating power of the Word, transmuting earthly inclinations into heavenly attributes and inspiring selfless service to humankind.
(The Universal House of Justice, from a letter dated 18 December 2014 to the Bahá’ís in Iran)