Fasting in general and voluntary fasting, in particular, is great worship. Fasting is not restricted to Ramadhan, but it is an act of worship that can be [and should be in some cases] performed at any time and at any place except when not recommended. Indeed, it is a worship that draws the believer closer to Allah and closer to perfection.

Fasting Mondays and Thursdays:
`Aa’ishah said: The Messenger of Allah, salla Allahu alaihi wa salam, used to fast Mondays and Thursdays”. [An-Nasaa’i; Sahih]

Abu Hurairah reported that the most the Prophet, salla Allahu alaihi wa salam, would fast be Monday and Thursday. He was asked about that and he said: “The deeds of people are presented to Allah on every Monday and Thursday. Allah forgives every Muslim except for those who are deserting each other. He says: “leave them for later.” [Ahmad; Hasan]

Intention for voluntary fasting
As opposed to Ramadan, the intention does not have to be made before dawn. The person can intend fasting [and start fasting] after dawn any time [even after noon] given that he did not eat anything. `Aa’ishah said: The Prophet, salla Allahu alaihi wa sallam, came to us one day and said, “Do you have any [food]?” We said “No”. He said: Therefore, I am Fasting”. [Muslim and Abu Dawood]

Fasting three days of every month(White days):
Abu Tharr Al-Ghefari said: “The Messenger of Allah, salla Allahu alaihi wa sallam, said “O Abu Tharr! if you fast three days of every month, then fast the 13th, the 14th, and the 15th [these are called the al-ayaam al-beedh, the white days]”. [Ahmad, an-Nasaa’i and at-Tirmithi; Sahih]

“Fasting and the Qur’an will intercede for the slave on the Day of Resurrection. Fasting will say: ‘O My Rabb! I prevented him from food and desires, so accept my intercession for him.’ And the Qur’an will say: ‘I prevented him from sleep during the night, so accept my intercession for him.’ He (sallallahu `alayhi wa sallam) said: ‘And they will (be allowed to) intercede.’” [Ahmad, at-Tabarani, Al-Hakim, Sahih]

So let us get closer to Allah by fasting Mondays and Thursdays or at least 3 days every month on the 14th,15th, and 16th. If we leave something for the pleasure of Allah then we will get MUCH greater in return!