your brain is a muscle. Like every muscle needs exercise so does your brain.Try stretching your brain by practicing every day a new word ,a new telephone number,a new address play a game like sudoka ,rummy. When you do deep thinking you will notice your face turns red from the extra blood flow you are sending to your brain thats called stretching.But dont overdue by getting a headache.
I concentrate very hard the first time I think about it for the purpose of storing it in my memory. Then I keep repeating it in mind over and over throughout the day or week or whatever.
Mnemonics also help. In other words, think of associations or hints such as something that you can spell with the Roshei Taivos of the thing you are trying to remember.
From the seforim, sadness, depression and anger affects memory. Too much olives too. But olive oil and happiness strengthens memory. That’s why Chabad is always humming nigunnim. They know.