An action cannot be regarded as an act of worship unless it meets two
conditions, namely perfect love and perfect humility.
ALLAAH 
says:
But those who believe, love Allaah more (than anything else)
Surah Al-Baqarah (2:165)
And they used to call on Us with hope and fear, and used to humble themselves
before Us
Surah Al-Anbiya (21:90)
If this is understood, then one will realize that worship is only accepted
from a Muslim who believes in Allaah alone (Tawheed), as Allaah
says of the disblievers:
And We shall turn to whatever deeds they (disbelievers, polytheists, sinners)
did, and We shall make such deeds as scattered floating particles of dust
Surah Al-Furqaan (25:23)
The Muslim's worship will not be accepted unless it meets two basic
conditions:
1- Sincerity of intention towards Allaah alone, which means that a
person's intention in all his words and actions, both outward and inward, is
to do them for the sake of Allaah alone and no one else.
And who has (in mind) no favour from anyone to be paid back, Except to seek
the Countenance of his Lord, the Most High
Surah Al-Layl (92:19-20)
And the Prophet {saw] said:
Allaah does not accept any deed except that which is done only for Him
and to seek His Countenance (pleasure).
Narrated by al-Nasaa'i, 3140;
classed as saheeh by Al-Albaani in Al-Silsilah al-Saheehah, 52.
2- The action must be in accordance with the laws which Allaah has
enjoined and stated that He must be worshipped only in accordance with them.
This is achieved by following the Prophet 
and the message he brought,
and not going against that or introducing new acts of worship or new forms of
worship which were not reported from him .
ALLAH Says:
Say (O Muhammad to mankind): If you (really) love Allaah, then follow me,
Allaah will love you and forgive you your sins. And Allaah is Oft-Forgiving,
Most Merciful
Surah Aal Imraan (3:31)
The Messenger of Allaah said:
Whoever does any action that is not in accordance with this matter of ours
will have it rejected.
Narrated by Muslim, 1718
i.e., his deed will be thrown back at him and not accepted from him. If
anyone goes against these two conditions (sincerity towards Allaah alone
and following His laws) in any action, then he will not benefit from it. The
one who seeks goodness and wants to please his Lord, let him worship Him and
draw near to Him by the means that He has ordained for them.
Source: www.islamqa.com/en
Taken from Parvez Shakir's Facebook page:
Worshippers Of ALLAH - THE ONE and ONLY.