الناس نسيان

The essence of man is forgetfulness. No matter how great or important a memory is, it will slowly disappear even in healthy people.

Unless the person is reminded. The antidote of forgetfulness is reminders. That is why we keep albums and photos. That is why we review before finals. That is why we use sticky notes for daily tasks. That is why it is so important to do ibadah, especially the five daily prayers and reading Quran.

Ultimately, man also forgets about Allah.

All of mankind acknowledged Allah as their Lord before this world, but the majority have forgotten. Even Muslims forget from time to time. Sin and discontent result from this forgetfulness: sin comes when man forgets Allah is watching, and discontent results from forgetting Allah's promise of eternal happiness in the next life.

Thus, we constantly need to remind ourselves that Allah is indeed the Lord of everything, that He is watching, that He will judge us. Regular dhikr and ibadah are very effective at this. We should also use other "sticky notes" such as observing and marvelling at Allah's creation, following Islamic accounts on social media, reading books and listening to lectures, and so on. 

الَّذِينَ يَذْكُرُونَ اللَّهَ قِيَامًا وَقُعُودًا وَعَلَىٰ جُنُوبِهِمْ وَيَتَفَكَّرُونَ فِي خَلْقِ السَّمَاوَاتِ وَالْأَرْضِ رَبَّنَا مَا خَلَقْتَ هَٰذَا بَاطِلًا سُبْحَانَكَ فَقِنَا عَذَابَ النَّارِ

"Who remember Allah while standing or sitting or [lying] on their sides and give thought to the creation of the heavens and the earth, [saying], "Our Lord, You did not create this aimlessly; exalted are You [above such a thing]; then protect us from the punishment of the Fire."
-Sura Aal-E-Imran: 191

It is also important to avoid sins and wasting time. This is especially for excessive and immoral entertainment, which combines both. Such acts fill our mind with things that cannot coexist with the remembrance of Allah, so we are forced to temporarily forget Him. And when one's mind is constantly deprived of Allah, the heart hardens and remembering becomes more difficult. Eventually, even reminders as bright as a full moon cannot pass through the heavy clouds covering the heart... until the day Allah has promised such people a dreadful punishment.

وَمَنْ أَظْلَمُ مِمَّنْ ذُكِّرَ بِآيَاتِ رَبِّهِ فَأَعْرَضَ عَنْهَا وَنَسِيَ مَا قَدَّمَتْ يَدَاهُ ۚ إِنَّا جَعَلْنَا عَلَىٰ قُلُوبِهِمْ أَكِنَّةً أَنْ يَفْقَهُوهُ وَفِي آذَانِهِمْ وَقْرًا ۖ وَإِنْ تَدْعُهُمْ إِلَى الْهُدَىٰ فَلَنْ يَهْتَدُوا إِذًا أَبَدًا

"And who is more unjust than one who is reminded of the verses of his Lord but turns away from them and forgets what his hands have put forth? Indeed, We have placed over their hearts coverings, lest they understand it, and in their ears deafness. And if you invite them to guidance -  they will never be guided, then -  ever."
- Sura al-Kahf: 57

To conclude, maximize the reminders regarding your faith, morals, and good habits. Minimize your sins and chances of permanently forgetting the Truth. Make sure to listen to some, if not most, reminders to benefit from them. For it is through constantly remembering Allah that true faith and success is attained.