In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Question to Imaam As-Sadi [rahimahullaah]: What are the means and deeds by way of which reward [on acts of worship] are increased?
Answer: As for the increase of reward for a good deed to ten times the like thereof, this occurs in every righteous action, as Allaah [The Most High] said: [مَن جَآءَ بِٱلۡحَسَنَةِ فَلَهُ ۥ عَشۡرُ أَمۡثَالِهَاۖ – Whoever brings a good deed shall have ten times the like thereof to his credit]. 
As for an increase beyond this, – and that what the questioner intends [in his question]- it has its means. It can either be related to the performer of the deed or the deed itself, or the time and the place [i.e. a particular time and place mentioned in the authentic texts that reward for good deeds are increased in that time period or place].
And one of the most important means of increasing [reward for good deeds] is when a person fulfils sincerity to Allaah and adheres to the path of the Messenger [sallal-laahu-alayhi-wasallam]. This [reward occurs] when a deed is legislated [i.e. in the Sharee’ah and not innovated] and the person intends to please Allaah and hopes for His Reward; when this [sincere] intention is firmly established as the drive and goal behind the deed because of his belief in Allaah, belief in the Messenger, [obedience] to the command of Allaah, seeking Allaah’s Countenance and Pleasure, just as it has been mentioned in a number of Aayaat and Ahaadeeth. Allaah [The Most High]: [إِنَّمَا يَتَقَبَّلُ ٱللَّهُ مِنَ ٱلۡمُتَّقِينَ – Verily, Allah accepts only from those who are Al-Muttaqun] - Meaning: Those who fear Allaah whilst performing deeds with Ikhlaas and Mutaaba’ah [adherence to the authentic Sunnah of the Messenger].
Also the Messenger [sallal-laahu-alayhi-wasallam] said, ‘’Whoever fasts the month of Ramadhaan, his previous (minor) sins will be forgiven”; ‘’And whoever stands [in the night prayer] during Ramadan out of Imaan and seeking reward [from Allaah], then all of his previous [minor] sins will be forgiven”  and other texts besides these. And because of this, the apparent actions have more virtue over one another in the sight of Allaah due to the Imaan and sincerity established in the hearts.
Also abandoning the forbidden desires of the soul is included amongst those righteous actions that have virtue over one another due to the different levels of sincerity- if a [person] sincerely abandons them from the bottom of his heart and not out of other than sincerity. The story of the companions of the cave [i.e. in Soorah Kahf] is a witness to this. 
Therefore, the means to increasing the [reward for good deeds] is that basis and foundation we have already mentioned – sound Aqeedah; a strong belief in Allaah and in His Attributes; a strong desire [to worship Allaah] and a fervent desire for goodness. That is because Ahlus Sunnah Wal Jamaa’ah in truth and those people of knowledge, – those who possess knowledge of the perfect and detailed of the Names and Attributes of Allaah and have a strong desire for the meeting with Allaah [in the Afterlife] – the reward for their deeds are greatly increased in a manner in which others who share the same religion can neither reach nor approach in the least. And this is why the Salaf used to say, “When Ahlus Sunnah are held back by their deeds [i.e. not having numerous deeds], they are supported by their sound creed” [i.e. the reward for their good deeds are increased due to been accompanied by strong Imaan, sound Aqeedah and adherence to the authentic Sunnah]. 
Imaam Ibn Rajab [rahimahullaah] said: The increase of reward [on a deed] is estimated in accordance with the perfection of a person’s Islaam and strength of sincerity in that action. 
[6:160];  [5:27];  Minor sins forgiven when one fasts the month of Ramadhaan, see article on this link:http://salafitalk.net/st/viewmessages.cfm?Forum=11&Topic=6255 ;  An Excerpt from ‘Al-Fataawaa As-Sadiyyah. Page: 43. Slightly paraphrased];  An Excerpt from ‘Al-Fatawaa As-Sadiyyah page 44 & Sharh Al-Asbaab Wal-A’maal Allatee Yudaa-afu Bihaa Ath-Thawaab page 59-65. Slightly paraphrased]  Jaami Al-Uloom Wal-Hikam 2/316]