Praise be to Allah.
It is human nature to be forgetful, as the Arab poet said:
He is only called man (insaan) because of his forgetfulness (nasiyaan), and it is only called the heart (al-qalb) because it changes so rapidly (yataqallib).
In the past they said that the first one to forget (awwal naasin) was the first man (awwal al-naas), meaning Adam, peace be upon him. Forgetfulness is something that varies from person to person according to each individuals nature; some may be more forgetful than others. Some of the things that may help to combat forgetfulness are the following:
Keeping away from sin, because the bad effects of sin result in a bad memory and the inability to retain knowledge. The darkness of sin cannot co-exist with the light of knowledge. The following words were attributed to al-Shaafa'i, may Allaah have mercy on him:
I complained to [my shaykh] Wakee about my bad memory, and he taught me that I should keep away from sin.
He said that knowledge of Allaah is light, and the light of Allaah is not given to the sinner.
Al-Khateeb reported in al-Jaami (2/387) that Yahya ibn Yahya said: A man asked Maalik ibn Anas, O Abu Abd-Allaah! Is there anything that will improve my memory? He said, If anything will improve it, it is giving up sin.
When a person commits a sin, it overwhelms him and this leads to anxiety and sorrow which keeps him busy thinking about what he has done. This dulls his senses and distracts him from many beneficial things, including seeking knowledge.
Frequently remembering Allaah, may He be glorified, by reciting dhikr, tasbeeh (saying Subhan Allaah), tahmeed (Al-hamdu Lillaah), tahleel (Laa ilaaha ill-Allaah) and takbeer (Allaahu akbar), etc. Allaah says (interpretation of the meaning): And remember your Lord when you forget [al-Kahf 18:24] Not eating too much, because eating too much makes one sleep too much and become lazy, and it dulls the senses, besides exposing one to the risk of physical diseases. Most of the diseases which we see result from food and drink. Some of the scholars have mentioned certain foods which increase the memory, such as drinking honey and eating raisins and chewing certain kinds of gum resin.
Imaam al-Zuhri said: You should eat honey because it is good for the memory.
He also said: Whoever wants to memorize hadeeth should eat raisins. (From al-Jaami by al-Khateeb, 2/394).
Ibraaheem ibn [sth. omitted] said, You should chew resin gum, because it gives energy to the heart and gets rid of forgetfulness. (From al-Jaami by al-Khateeb, 2/397).
As they mentioned, too much acidic food is one of the causes of laziness and weak memory.
Another thing that can help the memory and reduce forgetfulness is cupping (hijaamah) of the head, as is well known from experience. (For more information see Al-Tibb al-Nabawi by Ibn al-Qayyim). And Allaah knows best.