The du'a made at Tahajjud is like an arrow that does not miss it's target.
.
The following acts are recommended for one who wishes to perform the Tahajjud Prayer:
Upon going to sleep, one should make the intention to perform the Prayers. Abu Ad-Darda' quoted the Prophet (peace and blessings be upon him) as saying:
...
“Whoever goes to his bed with the intention of getting up and praying during the night, but, being overcome by sleep, fails to do that, he will have recorded for him what he has intended, and his sleep will be reckoned as a charity (an act of mercy) for him from his Lord.” (An-Nasa'i and Ibn Majah)
On waking up, it is recommended that one wipes the face, use a toothbrush, and look to the sky and make the supplication which has been reported from the Prophet (peace and blessings be upon him).

.

Tahajjud Prayer does not entail a specific number of rak`ahs that must be performed, nor is there any maximum limit that may be performed. It would be fulfilled even if one prayed just one rak`ah of Witr after `Isha’.

Samurah ibn Jundub (may Allah be pleased with him) said:
“The Messenger of Allah ordered us to pray during the night, a little or a lot, and to make the last of the Prayer the Witr Prayer.” (At-Tabarani and Al-Bazzar)