'Sunnah' (optional) Acts of Ablution:

The following acts are not obligatory, yet, once done, are rewarded for since they constitute the way the Prophet used to perform ablution:
Reciting 'Bismillah' (saying: 'Bismillahir-rahmaanir-raheem', which means: 'In the name of Allah, the Most Merciful, the Most Compassionate).

Washing the hands, three times, up to the wrists.

Brushing the teeth with a tooth-stick (Miswak).

Rinsing the mouth, thrice.

Sniffing water into the nostrils, thrice.

Combing the beard with the fingers (this is called Takhleel in Arabic).

Passing the fingers of both hands against each other, and also passing them through the toes (the Takhleel of the fingers and toes).

Washing each limb, three times.

Wiping the whole head with wet hands once

Wiping the two ears (inside and outside).

Performing the Wudhoo' in the proper order (as stated above).

Washing each part, one after the other, without pause, to ensure that no part dries up before the next step.

Humraan, the freed slave of 'Uthmaan bin 'Affaan, may Allah be pleased with him, narrated that 'Uthmaan, may Allah be pleased with him, once asked for a tumbler of water in order to perform ablution; when it was brought to him, he poured water from it over his hands and washed them thrice, and then put his right hand in the water container and rinsed his mouth and washed his nose by putting water in it and then blowing it out. Then, he washed his face thrice and then his forearms up to the elbows thrice; then, he passed his wet hands over his head and then washed each foot thrice. After that, 'Uthmaan, may Allah be pleased with him, said: “I saw the Prophet performing ablution like this of mine, and he then said: “If anyone performs ablution like that of mine and offers a two-Rak'ah (units) prayer during which he does not think of anything else (not related to the present prayer) then his past sins will be forgiven.””

Actions that require ablution as a prerequisite:

There are three actions that require ablution as a prerequisite in order for them to be accepted Islamically. These are:

1- Any type of prayer (Salah)

This involves only obligatory, voluntary, or funeral prayers. This is based on Allah's statement (which means): {O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles.} [Quran 5:6]

Also, the Messenger of Allah (may Allah exalt his mention) said, “Allah does not accept a prayer from a person who is not in a state of purity. Nor does He accept charity from misappropriated booty.” [Muslim]

2- Circumambulating the Ka'bah

Ibn 'Abbaas, may Allah be pleased with him, reported that the Messenger of Allah said, “Circumambulation is a type of prayer, but Allah has permitted speaking during it. Whoever speaks during it should only speak good.” [At-Tirmithi and others]

3- Touching a copy of the Quran

Abu Bakr Ibn Muhammad related from his father on the authority of his grandfather that the Prophet sent a letter to the people of Yemen, which stated: “No one is to touch the Quran except one who is purified.” [An-Nasaa'i and others]