Laylatul-Qadr is the most blessed night. A person who misses it has indeed missed a great amount of good. If a believing person is zealous to obey his Lord and increase the good deeds in his record, he should strive to encounter this night and to pass it in worship and obedience. If this is facilitated for him, all of his previous sins will be forgiven.
It is recommended to make a long Qiyam prayer during the nights on which Laylatul-Qadr could fall. This is indicated in many ahadith, such as the following - Abu Dharr (radhiallahu 'anhu) relates, "We fasted with Allah's Messenger (sallallahu 'alayhi wa sallam) in Ramadhan. He did not lead us (in qiyam) at all until there were seven (nights of Ramadhan) left. Then he stood with us (that night - in prayer) until one third of the night had passed. He did not pray with us on the sixth. On the fifth night, he prayed with us until half of the night had passed. So we said, 'Allah's Messenger! Wouldn't you pray with us the whole night?' He replied:'Whoever stands in prayer with the imaam until he (the imaam) concludes the prayer, it is recorded for him that he prayed the whole night,'…"
[Point of benefit: Abu Dawud mentioned, "I heard Ahmad being asked, 'Do you like for a man to pray with the people or by himself during Ramadhan?' He replied, 'Pray with the people' I also heard him say, 'I would prefer for one to pray (qiyam) with the imam and to pray witr with him as well, for the Prophet (sallallahu 'alayhi wa sallam) said: 'When a man prays with the imam until he concludes, it is recorded that he prayed the rest of that night.'" [Masa'il]]
Abu Hurayrah (radhiallahu 'anhu) narrated that the Messenger (sallallahu 'alayhi wa sallam) said, "Whoever stands (in qiyam) in Laylatul-Qadr (and it is facilitated for him) out of faith and expectation (of Allah's reward), will have all of his previous sins forgiven." [Al-Bukhari and Muslim; the addition "and it is facilitated for him" is recorded by Ahmad from the report of 'Ubadah ibn As-Samit; it means that he is permitted to be among the sincere worshippers during that blessed night]
It is also recommended to make extensive supplication on this night. 'A'ishah (radhiallahu 'anha) reported that she asked Allah's Messenger (sallallahu 'alayhi wa sallam), "O Messenger of Allah! If I knew which night is Laylat ul-Qadr, what should I say during it?" And he instructed her to say:
"Allahumma innaka 'afuwwun tuhibbul 'afwa fa'fu 'anni - O Allah! You are forgiving, and you love forgiveness. So forgive me."
Abandoning Worldly Pleasures for the Sake of Worship
It is further recommended to spend more time in worship during the nights on which Laylatul-Qadr is likely to be. This calls for abandoning many worldly pleasures in order to secure the time and thoughts solely for worshipping Allah. 'A'ishah (radhiallahu 'anha) reported:
"When the (last) ten started, the Prophet (sallallahu 'alayhi wa sallam) would tighten his izar (i.e. he stayed away from his wives in order to have more time for worship), spend the whole night awake (in prayer), and wake up his family."
And she said:
"Allah's Messenger (sallallahu 'alayhi wa sallam) used to exert more (in worship) on the last ten than on other nights."